So long story short my old lover is getting close with a friend of mine (not a best friend but been mates for several years). Feel really frustrated about it but not sure why nor how to approach the situation?

We have the same friendship group and split up about half a year ago on mutual agreement to call it quits . Both of us had dated other people since.

Logically brain is telling me to not give a fuck since its not like I'm in a position to date her myself due to upcoming plans but really annoyed that it's my fking mate? Feel betrayed tbh by both of them, especially since there was a verbal agreement not to sleep with each others mates.

Any advice on how to deal with this or appropriate things i can do will be very appreciated.