i think ur friend( read U) should go to his gf and let her know how many did he banged before and tell his side of the story. how many affairs he had and what was the reason behind the break ups and stuff, so that sge will have better understanding. and then its upto her if she wants to share her side of story. if she opens up thats her will, if she doesn't (assuming both guys really love each other) there isn't anything that u have to know. she is trying to forget her past and move on, so STFU and be next to her.
also there is a chance that she don't wanna say coz she couldn't remember how many😬😬