I'm not an expert, so if one of the guys out there with a psyche background wants to correct me, feel free.
I took human sexuality a few years back when I was racking up college credits for fun while waiting on a security clearance in the Air Force. One of the things that came up was the Kinsey Scale.
According to this scale, human sexuality is not a binary. That is, people aren't either heterosexual or homosexual, but rather fall somewhere on a scale between the two extremes. While small fragments of the population are exclusively hetero and homo, the vast majority of us fall somewhere in the middle. It is due to social conditioning that we choose one way or the other.
In the past, homosexuality has been a serious taboo, even punishable by death (and still is in some parts of the world) so it is likely that only those on the far extreme of the scale towards homosexuality chose that lifestyle. However, in recent years, this lifestyle has become more accepted. It's not a stretch to say that more and more people who feel even slight tendencies may be willing to give the lifestyle a chance as there are no longer severe consequences and there is even a greater opportunity for acceptance if one feels even slight alienation from whatever is "mainstream."
That being said, in today's culture it is highly unlikely that this guy is exclusively gay. VERY few people actually are. He may have only been with guys in the past, but if he and your girl feel a strong connection to one another, one of them may make a move, and the other may reciprocate. Especially if they are already going so far as to share a bed together (Which is inappropriate no matter what his orientation... Hell, I wouldn't even share a bed with my own sister when the only other place to sleep was a concrete slab floor.)
I would tell her to cease and desist. If she doesn't do it, walk.