What is it exactly you are worried about? Alcohol? Drugs? They will do that anytime of the day if they are going to. Sex that happens in the daylight too. Do you know her Boyfriend? Is he responsible? Do you trust him. I raised 5 boys and if you trust her all will be fine. I would talk to the host parents tho and find out details. My sons GF's stayed over but they slept on the couch and the boys were downstairs. WE had an open door policy, door must be open and mom can come in anytime to see what you are up to. If you don't find the information to be what you hope for then nix the idea.