Do you know ths guy? I think you should talk to him, tell him it was his girlfriend's idea. See if he's up for it (so to speak), and set rules about it (eg, how long for, extent of the kiss, where are hands alowed, where are they not allowed etc). Doing it at a party, in a room full of people might make him feel realyl uncomfortable. Maybe in another room (and after the kiss you could leave the two of them too it... or stay behind).
What ever you decide make sure that everyone involved, is comfortable beforehand, that way you'll all be more relaxed during it.
(and then tell us all about it)