What would you do..?

Okay so my fiancé told me that his best friend told him he was texting another girl, had been for a while and didn't want to be with his gf anymore. The texts I haven't seen but as far as I know they are quite flirty and have some potential in them to go further. Now.. my fiancé best friend, me and his gf have become friends since I've got together with my fiancé, we hang out together, few drinks, talk quite often, she's a nice girl and I feel like she needs to know, I also know the girl that his best friend is texting we were in school together. I'm struggling on what to do, do I risk my fiancé loosing his best friend or do I tell her what's going on?