Ohhh

sorry ... . yeah that helps. I thought man this person has some serious emotional stamina if they can balance a relationship with two people going through treatment at once.
You mentioned being worried this person will get better for someone else. Honestly, they may get better, but from what I've read from other peoples stories it takes a very long time to treat an illness like this 3+ years for recovery solid, committed dbt individual therapy, group therapy/skills learning, your own individual therapy, couples therapy later. That's without addiction issues, etc to contend with.
My ex (ltr, engaged) went inpatient last month - wanted a romantic break to focus on mental health (this was awesome!) - but then wanted to date others (we've been polyamorous before but this seemed like a bad idea for our relationship and their focus), they also wanted to quit outpatient - I broke-up with them (last week) but wasn't sure if it took (confirmed yesterday that we are indeed mutually broken up)
I hope you see the contradictions in this person's behavior and the impulsiveness. You mentioned now it's the outpatient thing. Sounds like this could very well be the same pattern of sand bagging treatment. A person who's getting better is going to be making better decisions.