A few possibilities come to mind:
1. She still loves him despite what she has said and despite the abuse. This isn't that uncommon, believe it or not.
2. She has financial reasons for being afraid of the marriage ending.
3. She's worried about social status and/or how she will appear to others.

Could be other things too. Your best bet is to ask her directly. You two definitely need to have a serious talk about your relationship in general it seems. It might not be easy for her to let go, especially if my first thought of her still loving him despite the abuse is correct, but it definitely sounds like she should do so. She may need help from a therapist to really let him go.