I agree with Bogey he could be a danger, the room mate does not know either way.
SaltySeaDogs please tell your room mate this is not acceptable, you do not wish to share your home with her boy friend when she is away, at all.
As far as him staying while you are there, I think you both need to work out what is acceptable, and whether/what he should contribute etc.
He could contribute towards food or electricity without actually paying rent. I am assuming there are no issues with this?
My friend did this, she started dating a man who worked on an oil rig and so he literally moved in when not on the rig, she did not discuss with us. Very annoying. I think I said nothing (long time ago). I would not not make that mistake again.
At the same time my other room mate, gay male room mate, had his boyfriend round a lot too, but never without that roommate so I never felt bothered by it. Plus they never did a lot of cooking in etc (like me) where as the guy from the oil rig did cooking etc and was there when his girlfriend was not in, which I found really not very nice. The cooking thing was just mess in the kitchen etc but the main thing was just being in my own home with a man I barely knew.