I've faced this in the past, to various degrees. Even though I'm the stepdad, I had to tell my stepdaughter that this guy is a loser. I didn't force anything, but I did say keep your eyes open and if you ever need help or a place to sleep, we're always here for you. I did draw the line, though and say "but I'm not interested in helping that asshole continue to be a douchebag". In fact, I've had to do that twice. I've also had to say "my house, my rules".
It's firm, but it's compassionate. I'm always there for my kids/stepkids, no questions asked and I think I've proven that over the years.
Of course, I've only ever done that with the full support of my wife, so there's that.