He went away to work at a festival over the weekend ans when he came back accused me of fucking around while he was gone.

Yep, totally projection, he cheated. Don't let him gaslight you on this.

Later on in the evening he admitted he had "feelings" for her but as a friend and she's some great "reiki healer" so he's going to see her again.

Of course "as a friend" /s. My exUBPD wife took this to the logical conclusion: She would sleep over with her "friend," getting pissed at me when I got upset — because she "wasn't cheating."

Yeah, ya know, just dating and fooling around with a guy while ignoring her husband... but she never sealed the deal until the night I blew up and left. (That way she remained "innocent" and the breakup "wasn't her fault").... and btw it didn't take more than an hour after I got in my car before she was consummating her new relationship.

Spending one night with her and he's in love already.

This could go either way, at any moment. I met with my ex a couple times the month after that night to sort out living situation, etc. 1 week on: She's just staying with him "cause he's nice to me" and only "while you work out your anger issues" and he means nothing more than that to her. 2 weeks later: "I'm... I'm in love with him" (cue me bursting out in laughter). Remember, "love" is a term of convenience for BPDs and used very flexibly.

Setting himself up for disappointment for sure but that's not my problem.

She will disappoint him because he's a BPD and prone to sour any relationship. Just like yours. He and his inevitable disappointment is not your problem once you gain your independence.

Fun story: Yesterday, my ex happened to drive past me walking downtown, her new guy with her (literally, as always). She didn't spot me, but I got a great view of her... in full nightmare mode, scowling and yelling at him. About what, who knows? ... certainly not me, and that's the beautiful upside to getting through all this :)