When easy child 2/difficult child 2 & her first boyfriend broke up (messily) I took her shopping for clothes. Her ex-boyfriend had blogged publicly about how she had the maturity of a 12 year old and the boobs to match, so we specialised in frilly, pretty blouses that showed off her cleavage. As a result, when he came crawling back after a few days, she had her self-esteem indepndently bolstered and was able to tell him, "No, I think you made the right decision in breaking up with me. YOu showed maturity and wisdom, we need to follow through and stick to your decision, take some time apart from one another for a few weeks."
Celebration is always good. Just avoid saying anything bad about him; even avoid agreeing with her because if by chance they get back together, she will tell him HER version of what you said. We went through that with sis-in-law - and now her husband avoids spending time with us.
Marg