Is my friends girlfriend cheating on him?

My friends girlfriend is naturally quite flirty but I've noticed recently that shes been flirting a lot with her housemate, and they'll speak in whispers to each other and cuddle up on the couch when watching tv if my friends not there. Also when Im over there and playing on the PS3 with my other friend theyll both go up stairs for quite a while at the same time and when I joked to her housemate that he liked her once he got very defensive and said they were just friends. What do you think is going on?