How would they react to their friend going nonverbal in the middle of hanging out?
(The friend would try to apologetically explain in writing, if asked what's going on.)
Undertale Sans - You don't even need to write, Sans noticed. He lets everyone walk past both of you, and he slows down to walk next to you, far from all the people talking that are clearly overwhelming you. If you need a break, he's more than happy to sit with you for a little while. Sans gets overwhelmed quickly as well when he has to socialize, so you're actually also giving him a small break from that.
Undertale Papyrus - That's fine, he can talk for both of you! Papyrus gets it a lot actually. He has autism, he's socially awkward and he gets burned out really fast when he has to mask the fact that he's so tired of talking to people. He can do the talking for a little while until you feel better and ready to talk again!
Underswap Sans - Blue can tell something is bothering you, and does his best to more or less discreetly take you on the side to check on you and be sure you're alright. Going non verbal is fine by him, he's used to it with Honey already. He just tells you to pull his sleeve if you need something. He's always happy to help.
Underswap Papyrus - Honey noticed you're tired, and he slowed down as well. He's tired too anyway, he hates to being forced to socialize for more than a few hours. When he sees an opportunity, he turns chaotic gremlin mode and drags you away so you two can enjoy doing something else while the other keeps doing whatever they're doing. Let's watch a movie or grab something to eat and play dead while the others start panicking wondering where you went. You and him are not available anymore today.
Underfell Sans - Red noticed you're not talking anymore and that's sort of stressing him out because he wants to help but he also doesn't want to call you out in front of the others as, well, that's a weakness, you never know, what if they randomly attack you? Sure, they're supposed to be friends, but if you knew the number of "friends" that suddenly stopped to be once they reached their goal... Red eventually grabs your hand and stops walking, silently asking if you're ok. He can take you home if you don't feel like hanging out together. It's not like any of the other notice you're not alright anyway, so who cares about ditching them all?
Underfell Papyrus - He's trying to be sneaky, but he is definitely walking in front of you to hide you from the other people of the group, so they don't noticed you're not ok. He's doing it completely instinctively, and will actually tense every time someone looks your way. He doesn't call you out, but if you show any sign of distress, he's ready to just lead you somewhere calmer so you two can relax in silence. He doesn't mind.
Horrortale Sans - Oak takes your hand and starts purring low, hoping to soothe you a little. Oak has a very small social battery, he's just as overwhelmed and he would be more than glad to just sit for a few hours in a café or something. He's tired of walking, he's tired of hearing people talk and he doesn't like that you're clearly not having a good time. He will definitely try to manipulate you a little into sitting in a café by staring with hopeful eyes at any café you walk past, then at you, then at the café again, until you realize and follow him. He's very proud of himself.
Horrortale Papyrus - Willow immediately lowers his voice and signs to ask you if you're alright, not wanting to call you out. Depending on your answers, he will either act like nothing happened or happily cut the hang out pretending he has other things to do and invite you to come along. He's adapting. Besides, Pumpkin came along and Pumpkin is clearly tired already, so you would do the guy a huge favor by just giving up.
Swapfell Sans - Nox keeps an eye on you but doesn't call you out, instead working to keep everyone out of your personal space so you don't feel overwhelmed. He will regularly check on you and at the slightest sign you have enough, he's offering a ride back home so you can chill on his couch instead.
Swapfell Papyrus - Rus stops clowning around as he notices you're starting to get overwhelmed and simply walks next to you, holding your hand. He's trying to cheer you and hype you so you continue to enjoy the hang out, but he's also ready to randomly shortcut you home without telling everyone to chill on his couch if he sees you're not ok.
Fellswap Gold Sans - Wine doesn't hesitate one second to tell the entire group to shut up as it's clear they're talking too loud and that you're overwhelmed. Well, that's awkward now and sort of everyone else, especially as he's not acting like some sort of teacher, shushing everyone rising voice again. Hopefully it's over soon...
Fellswap Gold Papyrus - Coffee understands immediately and switches to non verbal as well to make you more comfortable. He lets you cling to him and is clearly looking for an escape (and an excuse to just hang out with you and cuddle). Eventually, he drags you to a movie theater and you do nothing but eat pop corn and cuddle for two hours :D