This is a huge topic, Woodie. People get stuck in self-destructive behavior patterns for a lot of different reasons. The case you've described sounds similar to the child of an alcoholic who marries an alcoholic even though s/he says: Never! No way!

People are attracted to what's familiar, and often they don't know how to relate to people who aren't abusive or controlling. Your friend may find a nice, supportive, non-abusive guy boring or just plain perplexing. It's hard to relate to a non-abusive person in an intimate way if you've never learned the rules and expectations of a healthy relationship. Your friend may not even know how to recognize the red flags of a potentially toxic relationship where she's able to say--Nope, not going there this time. Then, before she knows it, she's in over her head. Abusive, controlling relationships are a like a tornado. They can suck a person up and throw them a hundred miles off course in a matter of minutes.

It takes a lot of work to break out of these patterns, as well as a lot of personal support. What they say in AA is that "insanity is doing the same thing over and over again, but expecting different results each time." There's an interesting parellel between abusive relationships and addictive, self-destructive behavior.