She could be doing it for attention. Maybe she's emailing you about her conquests to get a reaction out of you. She wants you to go "oh my god, that's so cool, tell me all about it" or something similar. I've known girls who pull or sleep with guys basically because she thinks he/others will like her. It also gave them a confidence boost to think that guys would sleep with them. The best thing to do is just ignore her and soon she'll shut up about it. One of my old friends said to me one day "Oh my god, I pulled 4 guys last night" and she was expecting me either to be jealous or impressed. I didn't give a ****. I had a boyfriend and was much more proud of that than having to go around shoving my tongue down randoms throats.
Then again she might not realise what she's doing. She might not realise that she's upsetting her friend by snogging her friends ex. The word EX just gives some people the go ahead no matter who is involved. When it comes to the opposite sex, some people can be VERY selfish.