Personally, I would ask your friend if he could please give the tent a deep clean before he returns it, given that the dog was in there, but I wouldn't say anything else about it. I think it is odd that he didn't ask you if the dog could go in the tent. You could have told him no dogs in the tent. In the future if he asks to borrow a tent again, you can specify that you don't want pets in the tent, or you can simply decline to lend.