He needs his own space but I think it should be noted that this say when he is over, How often is he over? Otherwise if they see each other infrequently, like visiting an uncle rather than living with a parent as is the norm in the U.S. It may be the equivalent of sharing a tent while camping.
This is also Posted on the Teen parent site where I have answered it, I said No he needs his own space and Privacy as he will be in Puberty soon, if he is scared find out why and have a chat with your Fiancee. A 13 year old needs his independance, make sure nothing has happened to make him so fearful and ask him how he feels and why he is scared. Make sure he has a nice room and your Fiancee needs to let him go to bed each night by himself.