you will need a female accomplice for this, but it would be a good one...
if you can find the line of sight from the gf's perspective, borrow a bra from the chick and drape it over something in that line of sight., then have the chick come in, grab the bra, and be all like "there you are! you must have thrown it there when you ripped it off last night, (roomates name). we going to hang out again tonight?"
his gf will be so confused, pissed, etc... and you wont have to show off your shortcomings by beating off infront of a strange girl.