3. She invites him to go on double dates.
After you tell your girlfriend that your boy is in love and very happy in his relationship, she wants to celebrate (or investigate) the new happy couple.
While she usually nags you about being more romantic and doing things with just the two of you, she insists that the lovely birthday dinner you have planned for her be a double date with your ace and his special plus one. When you get to dinner, though, she side-eyes dude's main squeeze and then directs her gaze at your friend all night.
If you already have a few signs that you should be concerned, and the dessert arrives and she insists on sharing hers with him, it might be time to grab the bill and have a talk.