My wife’s ex girl friends name just showed up on our find me app why? Are we in danger of being hacked?

No. You are neither hacked nor are you in the process of being hacked. The two people probably shared their location in the past and used the Find My app to locate each other. If the other person's name just showed up recently then perhaps location sharing between the two was turned OFF at one point and then turned ON again. Not sure exactly how the Find MY app was set up between the two in the past therefore I am unable to provide a more specific response than the one above.

Axel F.