We took our DS15's friend with us last summer and it was a wonderful experience. This September we are taking another friend for him. In both cases it was/is the first time for each young man to visit WDW. Along with a notarized medical permission slip (which I agree with another poster that it would not be needed), we took copies of his insurance cards. I also asked his mom for a typed list of info that would be pertinent in case of an ER visit: full name, address, medical history, weight, height.
I gave them general info about what they would need to take as well as the amount. Last summer I also gave the mom a list of things we provide at the hotel, such as toothpaste, shampoo, sunscreen, beach towel. We made it clear what we were paying for and suggested an amount for them to take for spending $. I also told the mom that my guys are not interested is buying souvenirs so there would be no pressure for her son to spend much money.
In the case of both the boy who has gone with us and the one that will, they have stayed overnight with us many times and gone on short guy trips with my husband and sons. We knew they would be compatible to our way of life.