I'm guessing one of the reasons people believe the "FRIEND = Mike" theory to be plausible is because of that one line from the end of the SNEO encounter, if he was defeated by fighting, where he directly addresses Mike by saying "Are you getting all this [Mike]!?", which seems to imply that Mike is actually present, and that he might be FRIEND specifically, since FRIEND is seen in the basement.
However, to take the line at face value is missing the fact that Spamton is hallucinating during the encounter; he's experiencing a break from reality and hallucinating that there is an audience, and that this audience is taking his furniture.
And literally right before he addresses Mike, he actually recites his and Tenna's old jingle. More specifically, he recites his part and the shared part. To maintain the rhythm despite omitting Tenna's part, he repeats his own twice; Tenna does the same in Chapter 3:
(Note that "Turn Up The Juice!" is entirely in brackets, and it's specifically all in one bracket that repeats twice, implying that Spamton didn't alter the line. In both cases, the line ends with an exclamation point, while Tenna's first "Fresh from the Juice," ends with a comma, implying that his part comes first.)
Spamton hallucinating an audience, combined with him reciting the old jingle, implies that he's specifically hallucinating that he's in TV World again... which is where Mike was, as Chapter 3 revealed that he's some sort of producer/assistant on TV Time. Addressing him directly like that would come naturally to Spamton if he believes he's in TV World, since he used to be part of the show as the commercials guy (plus a contestant at least once; and according to his contract with Tenna, he was supposed to become a host), and he used to know Mike.
And outside of that one SNEO line where Spamton is clearly hallucinating, the way Spamton talks about Mike seems to imply that Mike is another lost connection of his, like the Addisons, Swatch and Tenna.
The "Easel" is Swatch, and the "CRT" is Tenna, while the meaning of "Man, Woman, or Child At Half Price" is unclear, but considering that none of the other things mentioned are random, maybe he's referring to the Addisons? The "half price" could fit, since they're salespeople and they claim they're selling their products at a big discount.
Anyways, he mentions Mike at the end and becomes sad upon doing so, which, combined with one of his Sweepstakes answers, is implied to be because he still views Mike positively and misses him. Out of all the connections Spamton lost, Mike is the only one who didn't betray him in some way (neither did Tenna, but Spamton doesn't know that yet); they just haven't seen each other since Spamton left TV World. This may also be why Mike is the only one Spamton refers to by name.
The Chapter 5 cutscene with Tenna and the fake Mikes revealed that the real Mike was trying to dissuade Tenna from giving Spamton the contract, because he had a "bad feeling" about it and suspected that something terrible would happen to Spamton if he signed. (As for how Mike knew that, my theory is that perhaps he noticed Spamton being nervous/tense around the phone and figured that whatever is on the other side probably shouldn't be messed with.)
He may have possibly tried to dissuade Spamton from signing as well. It would make sense, and it would also fit with the way Spamton references Tenna's "betrayal" right after reminiscing about Mike; he could be remembering that Mike had tried to warn him about the contract, which he in hindsight believes to be Mike's attempt to save him from Tenna, since he was too lovesick to realize he would get "betrayed".
Either way, Mike was trying to prevent the contract from happening, and it's implied that he was upset about its consequences, with him "acting different" since the "other guy" (Spamton) left. The phrasing of "acting different", along with Battat saying that sometimes when Tenna asks Mike to do something "it just sorta happens!", implies that Mike was still there, but perhaps wasn't talking to Tenna anymore. In the Chapter 5 cutscene, Tenna says that "since then... it feels like... you're not really here", and that it feels like playing pretend, so he implies that he knows he's playing pretend with the impostor trio, and therefore Mike's change in behavior likely isn't just explained by the difference between his personality and those of the fake Mikes.
Mike trying to prevent the contract and being upset about his worries coming true implies that he's benevolent towards Spamton and cares about his well-being. When you combine this information with Spamton implying that he's all alone, it's more likely that Mike is just some guy from TV World that wasn't able to follow Spamton to Cyber World and doesn't know what happened to him after he ran away, hence why he's so upset with Tenna for the contract.
Meanwhile, FRIEND is... some sort of strange entity with unclear motivations that can move between Dark Worlds on its own. It has been seen, or its presence has been referenced or implied, in Chapters 2, 3 and 4. Its appearance in Chapter 2 is the most notable here, since that's an instance of it appearing in Cyber World, where Spamton lived. The idea of Mike being FRIEND, and thus having the ability to be near Spamton, yet Spamton is still implied to be all alone, would go against Mike's known characterization as someone who cares about Spamton.
(This is also notable considering that Spamton does appear to have some connection to FRIEND. He wears the pink and yellow glasses, SNEO's design features even more of that color combo, and on the Normal Route the SNEO encounter occurs in the basement, where FRIEND also appears. He also speaks in all caps, the way "FRIEND" is written in all caps; the all caps text is a trait both of them also share with the introduction of Deltarune and with Jevil, who is a Shadow Crystal Boss like Spamton.)
And of course, the cherry on top is the fake Mikes. The trio references several incorrect fan theories, such as Mike being the Dreemurrs' TV (Battat asking if he's Tenna's alter-ego, and Small Mike's smile being similar to the one that appears on the TV in Chapter 2), the Woody Theory (Cowboy Mike), and the FRIEND theory (Cat Mike). Even the fact that they're treating Mike as some sort of big mystery to be figured out may itself be poking fun at fans who do the same, when Mike is actually just some guy.
But one thing the fake Mikes have in common is that all their designs feature microphones (Small Mike and Cowboy Mike's heads; Cat Mike's hands and feet), which they likely got from the microphone symbolism in the Mike Room. It's not simply them mistaking the name "Mike" for the word "mic", since their dialogue spells it correctly.
TL;DR: I don't think Mike is FRIEND. I'm inclined to believe that he's probably the realistic microphone; its sole appearance has it fading in and out, implying the ability to become invisible and/or teleport, which would explain how the real Mike has remained unseen by the impostor trio despite being present.
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