Question ladies

Me and a guy friend of mine was discussing this. Let’s say your girlfriend gets close with a girl that he used to date. The girl knows that she dated the boyfriend but doesn’t tell the girlfriend (it was years ago). Should they tell her or not tell her? My opinion they both should say because I don’t want to be all up in this girl face and she slept with my boyfriend I would rather know then find out or I would be really pissed then! Y’all let me know what y’all 🤔think