To me, the thing to do is accept that your partners have a sexual past and move on.

Additionally, in this situation she was 15, it was at least seven years ago, people change.

Finally, be aware that many people do not leave abusive relationships at once, for a variety of reasons. This may seem unfathomable to you, but often it is in fact the case. This does not mean that they want the abuse. You could research this topic further online.