maybe just tell him? but break it to him slowly? Like 'Hun we need to have a chat about 'sis', she is going to make an announcement on the weekend that may come as a shock to you' then depending on his reaction tell him. But tell him he has to act surprised yet be happy for her!

At the end of the day, it will not matter what your partner says about her BF - she will do as she pleases anyway until she herself has had enough.

I hope things go well and he can put aside his issues with her partner tobe happy for her