I feel for your friend. I don’t know how it is with them, but for me this dynamic used to occur because it was behavior I had seen and considered to be normal relationship behavior. We humans learn by what we see, and a lot of what women see, especially in “romantic” movies, is an unhealthy dynamic where the woman has to sacrifice and the man gets to live how he wants. So while it might be simple, the psychological bonds keeping people in these situations are layered and complex.