This has nothing to do with all this beta or alpha bull crap, but it does have to do with you being so in love that you’re being purposefully blind and making excuses.

There’s always HR, there’s leaving the job (even if it requires moving away. If the situation is as bad as she says, she’d most likely do it) there’s properly ending the relationship with you. Usually I’m inclined to believe women when it comes to things like these, but her story sounds kinda sketchy to me. Regardless, at least the first time was just old fashioned cheating. Is he her boss or “just” a coworker? There’s ways for her to collect proof of what he’s allegedly doing, but she’s not doing that either. Things like recording conversations or getting him to put anything in writing via text. Having someone “accidentally” overhear a conversation where she expresses her concerns to him about the situation again.

Do you think she is in physical danger? If yes, then it’s time to convince her to become more proactive and put a stop to this.

If she is telling the truth, then she’s still choosing to stay in the situation and not change anything. That means there’s nothing you can do but look out for yourself now. It’s not fair of her to expect you to endure the constant betrayal and hurt. End it asap. Remove yourself from this dumpster fire. I’m almost sure you’ll see her announce a relationship with him soon enough. But I’m kinda jaded Ig.

You deserve better. Be strong and think of yourself in this first. You’ve got this! If she reaches out for help in a platonic capacity later on, you can re-evaluate if you want to help her in any way that doesn’t cross your boundaries. But NC or very low LC might be best for now, if you decide to leave.