I think you handled this wrong. This girl could have a crush on him and you decide to back off and let him go alone without you.. you should be going with him and trying to be friends with these people too. You only met her twice so you should have proved her wrong and showed her your not boring at all.
Show them how strong a couple you are and ignore her stupid comment. Not being there is allowing her time to try and get inside HIS head. You should be more possessive and turn up with him looking like a model, holding his hand, smiling and secretly saying f**k you to that bitch
"Don't ask a question if you can't handle the answer".