Kid going away with friends

My 9 YO has been invited to go about 12 hours away for an entire week
with the family of a friend of his. This friend is a good friend, but
not someone he spends a ton of time with. Two overnight get-togethers
have gone well. My son is very sociable but also gets cranky and needs
alone time on a regular basis. The family is not an issue; I trust
them safety-wise, and they're also nice people. However, 9 seems too
young to me to have a child be that far away from home for an entire
week, and I worry about him getting sick of the family (his friend and
two younger sisters, mom, and dad) half-way through the week.

Problem is, the place they're going is a great place to go, both fun
and educational. I feel guilty about not letting him go, both because
I know it would be a great place for him to go (not that that means he
has to go this year) and because the other mother really wants him to
go for her son's sake.

Any thoughts? Are we just being babies about our "baby"?