Well actually I have no problems with what Alexis did at the moment. Basically Erynn is expecting from her friend that she stays away from a guy that Erynn is not in a relationship with, the guy has confirmed he is not in a relationship with her nd has not seen her or heard from her for 5 years and has never been in a romantic relationship with her as such. The fact that Alexis herself is still torn by her loyalty to her friend speaks well of her, but Erynn is laying claims where she has no claim.
My problem with Alexis didnt come from any "cheating" angle. It came from the fact that she took advantage of MC.
She learned pretty fast who he was. She never gave herself away, she kept doing it after MC landed. She knew exactly what she was doing.