How to help a friend in a toxic relationship?

I’m really concerned my friend is in a toxic relationship and I don’t know where else to post. He is 22 and she is 30. He’s a sweet, young guy who has a people-pleasing tendency. He’s also a bit co-dependent and loves to be the rescuer (his words).

Their relationship has always been rocky. They’ve been involved for a year and a couple months and have been broken up/not officially dating/in a rough patch for the majority of that time in this on-again, (mostly) off-again cycle. When they first started dating it was especially touch-and-go with neither fully committed to each other. During an off-period, they both saw other people. But when his GF found out about it, she used it against him and guilted him. And then, after they decided to seriously date, just a mere 2 or 3 months later, he found her in bed with another man and was devastated and traumatized. But he decided to forgive her because he always makes excuses for her behavior by saying he’s made mistakes in the relationship, too. And he sees her emotional issues as his responsibility (the rescuer/co-dependence trait) and can’t seem to separate taking care of himself and taking care of her and conflates the two.

He’s confided in me a lot about the issues in the relationship and has told me I am not the only friend who has had concerns about this either.

He’s said to me that he feels like a masochist for staying in the relationship, he can’t be fully honest with her and knows that’s problematic, he says she’s draining, that she takes things out on him unfairly, he says that he feels he should’ve left the relationship a long time ago, he says he doesn’t know what he’s getting out of it anymore…but then runs to her rescue (she lives out of state now) at the drop of a hat; cancelling plans on friends, forgoing commitments to his work, and compromising professional opportunities. He says he knows he needs professional help, that they’re trauma bonded, and that he has this rescuer streak; he talks about how unhealthy and problematic the relationship is; but then he turns his back on all his commitments to dive 200% back into the rollercoaster of the relationship at the flip of a switch.

I’ve seen it take such a toll on him, and he knows it takes a toll on him. He’s told me how he wants out, but that he loves her. And he’s just too young to understand how toxic it is, how that toxicity affects other areas of his life, or how to leave it. And it’s breaking my heart to see him in this cycle.

It’s stressful to see my friend in such an unhealthy dynamic and feel like there’s nothing I can do. And now it’s affected our relationship, too. His inconsistent behavior, among other things, has eroded the trust we had. He was my most reliable, most meaningful, and closest friend and now I never know what plans he’ll cancel to leave the city or when he’ll come back, I don’t know if I’ll hear from him while he’s with her (like I said, he tends to ignore his other commitments/relationships and concentrate fully on her while he’s with her), I can’t trust him, and I don’t want their toxicity affecting me. I just feel like I’m losing my best friend to this toxic relationship and I’m sad A) to lose my friend and B) not to be there to support him anymore.

I feel like I need to step away, but we also have so many mutual friends and were in the middle of working on projects together. And I’m worried that if he doesn’t have me as that confidant in his life anymore, that he’ll get further wrapped up in this dynamic. I know I can’t be responsible for his journey and have to choose my own well-being first, but I’m concerned and would appreciate any advice or insight.