I'm wondering how they have the money to pay for this. I certainly wouldn't pay one penny for it. I agree that if they are 18 you can't really do anything about it EXCEPT like someone said, if you are supporting your son you certainly can forbid it. If you are paying they bills you have every right to say what he can and can't do. And if that's the case, I would say no.
About the grandma thing, you could be a grandma whether they go on a vacation together or not..
About the grandma thing, you could be a grandma whether they go on a vacation together or not..