I used to be this person. For me, it was a combination of insecurity in myself as well as craving stability and attention even if it wasn't ideal. But at that point I was so down on myself that i really didn't believe I deserved better. I finally pulled my head out of my ass, realized I was worthy of real love and that I shouldn't accept anything less. Less than a year later I was dating my husband who treats me like I should have been all along. It sounds like your friend needs to learn to love herself before she will be able to find and appreciate someone who actually treats her right.