Just play along and agree, it will make her laugh or confuse her. Either way, it makes you not look like a jealous jerk. She's with you for a reason, so unless she's making lots of plans with this guy individually and spending more time with him than you and is giving you bad vibes, don't worry. As I said, she's dating YOU, and its just as easy for her to leave you if she's disinterested.
My best friend's girlfriend always says how Im much more ideal for her, how Im her usual body type/personality/interest choice. Like, says this in front of him, or to him, with me there. But shes with him (and I dont want her), and he knows shes dedicated to him.
I totally understand your frustration (Ive had similar experiences) but try to accept that he's not a threat, until he makes a move obviously. She can and will leave you if she wants this guy, but if she is exhibiting no signs of this (outside of your paranoia)... take a couple deep breaths and just laugh it off.