I had a similar situation arise when I went on a backpacking trip to the grand canyon a few years ago. We had permits for 4 people and my friend who I hiked with a lot invited himself knowing that there was room. The problem was (as much as I like my friend) is he drink like a fish and is a major a$$H0!e when he drinks. After a couple of trips into the backcountry when he threw tempertantrums while drunk, I didn't want to spend a week in the bottom of the canyon listening to his $#!^. As the trip drew near he kept asking for details (after never being invited) and I felt I had no other option then to level with him. It sucked plain and simple. He was broken and couldn't see my point of view(because he was always drunk). I basically explained that this was a trip of a lifetime and I didn't want to spend it babysitting him.

He was pretty steamed but eventually got over it. I think he got over it because I wasn't the only one to point out to him he was out of control and no fun to be around when he was drunk (75% of the time).

Bottom line IMO if he's really a friend you need to level with him. He may not like it but he'll respect it more then if he catches you in a lie. Either way, don't expect it to be easy.