So sorry to hear about your friend's son. I would suggest that you look at having your friend make a referral to the Make a Wish Foundation. They have tons of experience with granting wishes like this and are experts at being able to work around details such as treatments, special needs, etc. Generous donors allow them to cover the costs of the trip. They go out of their way to make sure the trip is a special experience for the family. You can get all the information on their website.

My thoughts and prayers will be with your friend's family.