Grab her other boob and join in

You step up closer and cup her other boob. Startled, her head snaps up and she looks at you, you see her tense up a little and sense her knee heading toward your balls. Before she makes contact she stops and relaxes. Narrowing her eyes as if to say how lucky you are to get away with it, she smiles and puts her head back against the other man's chest, leaving you free to continue groping her.