I don't quite get what you mean by 'sit' - do you just mean, hang out?
So he sees her Friday evening and Saturday afternoon and evening?
Honestly, that doesn't sound that unreasonable - sounds pretty typical. I think it's good he's still seeing his buds. I would be concerned as well if he wasn't fitting in time to see his friends.
I always look to make sure there's balance. I think in the early stages of anything (any new friendship/relationship), people (kids and adults alike) go a bit ga-ga - don't they?
I know I have. I think I'd note unhealthy behaviors - like obsessive things or personality changes - but you don't mention that at all. This just sounds like typical teenage/young adult interest.
If it were me, I'd encourage him to have her around to your place, so it's not so one-sided. I think you would be much more comfortable that way - your mind would be put at ease. You should be getting to know her also.
To me, it sounds like her parents are getting to know, like and trust your son.
I would ask him to respect certain times (if you really don't want him coming in at all hours and it wakes you up, etc. then that's ok - it is your house). When I came home during the summers I think I was respectful of that.