It may not be illegal (depending on what the age of consent is where you live), but if you don't want your daughter possibly having sex or doing any other crazy stuff college kids are known for doing, I wouldn't let her go.
Well if shes going there to "sleep" over they are obviously going to be sleeping in the same bed together. They might not neccessarily have sex but they will eventually if you let her go after this first time.
I wouldn't even ENTERTAIN the idea....absolutely NOT.
My daughter will be 16 in 3 months....I don't even let her go to boys houses without parents there, AND i talk to them first.....
Only community rooms.....no bedroom and no being left alone.....
I know eventually kids figure out how to get away "alone"....but I'll put every stop to the situation I can....what I can control, I do.....maybe I'm too strict, but kids will be kids.....given opportunity, they WILL take it.
I was there once....no one had control over me, or cared really.....I had my first child at 17.
I wouldn't allow it unless your ok with the idea of your 16 year old daughter coming home pregnant one day or with an STD. College guys can get around. You do realize they will probably have sex this time or the next (if they have not already.)