Why do you think Rhea saved Beth? Do you think it was an act of betrayal towards Rio? do you think he would feel that way? I also wonder if she knew what really happened between Rio and Beth that night and if her decision would still be the same? I think her helping Beth was more about Rhea and her feelings and Marcus than actually looking out for Beth if that makes sense but what do you think?
I think Rhea saved Beth because she didn't want to be complicit in her murder. The show suggests that Rhea knows Rio well enough to trust Beth when she shares at soccer practice that she's both not pregnant and that if Rio knows, he will kill her, and the moment that happens, Rhea has two choices: she could either do nothing and allow Beth to get killed, or do the bare minimum and her own conscious could be clean. She chose the latter, and that makes sense to me because I think the show demonstrated that Rhea's world looks a lot more like Beth's former picture of suburbia than it does Beth and Rio's current entanglements in crime, guns, pills, and counterfeit cash. She's not part of that world, nor does it seem she wants to be, and I think she makes it fairly clear in the final scene where Beth chases after her and Marcus that she wants no further involvement—she got Beth spared, and it was up to Beth to figure it out from there. Simply put, I think most normal people walking down the street would have a hard time doing nothing if they knew that it meant that a real person—one that they cared for in any capacity at one point, even if the circumstances of that were revealed to be false—would be murdered as a result of that inaction.
The question of whether or not its a betrayal towards Rio is complicated, but my instinct is that overall, no, it's not.
First, I believe that canon suggests that Rio's intention to kill Beth is very real, but not truly what he wants, and ultimately he would have regretted it had he gone through with his original plan. In just one episode between 3.03 and 3.04 he learns that there's no baby, and he doesn't have the same drive he had upon their initial meeting. The cocktail of feelings and motivations driving his decisions, then, weren't something he could sustain, and I think it would be too 'black-and-white' to label it purely as a betrayal when that reality is factored in. I don't think Rhea necessarily foresaw that, but I do think Rio wouldn't hold onto it or categorize it as a betrayal in the long run if he knew.
Secondly, I simply don't believe Rhea thinks she owes Rio her complicity in his choice to commit violence, even if its in reaction to violence committed against him, which means I don't necessarily think Rhea would consider it a betrayal, either. Once she was dragged into their mess—entirely against her will, in all instances!—her action or inaction had real consequences that she would have to live with. I think ultimately she wishes she didn't have the problem dropped in her lap, but once it was there, I don't think she believed she owed Rio her participation in aiding his plans for murder.
I actually think Rhea's decision would be the same regardless of how much she knew about what happened in the loft. In the end, Rio was alive, and so Rhea and Marcus didn't actually lose him or have their own sense or desire for that type of revenge. I think she lost all respect for Beth and wanted nothing to do with her from that point forward, but I don't think that meant Rhea wanted her blood on her hands.
I think the more complicated part of a betrayal existing between Rhea and Rio is Rhea allowing Rio to believe that Beth is pregnant with his child more than doing what she can to spare Beth. However, the pregnancy lie is resolved so quickly that I'm not sure anyone would hold onto that betrayal for too long, particularly since Rhea couldn't reveal the truth without also further making herself complicit in Beth's death.