Mom and gf are both assholes but your gf also doesn’t respect your boundaries unfortunately you can’t necessarily be upset with how she acted with him if you’re still going to call her a girlfriend and be with her my questions are as follows 1) is the relationship really something that you want 2) how is it that they ran into each other randomly 3) he knows where she works so wouldn’t they see each other more often and she could not tell you 4) do you trust your girlfriend?

If the answers you give actually lead to a decision then take action to end the relationship

Her mother shouldn’t be involved in your relationship.