This type of situation can be a little awkward. While it would be wonderful if you could pay for the whole trip, there may be a case where that isn't possible. So, then comes the question that we seem to be divided on here. Don't invite, invite and pay for it all, or invite and offer to pay for a large part of the trip asking the parents to chip in some, too.
I think it may depend on the type of relationship you have with your son's friend's parents. In some cases, you know they wouldn't mind paying. In other cases, you may not be sure whether they can afford it at all. I think, if you approach the parents first and explain what you are thinking about doing, the costs involved and ... explain that you realize that it isn't always possible to come up with the money for something like this when it isn't planned ... but if they think it might be possible that you would like XX to come along. That gives them a way out and they may not feel awkward about saying no. And, if you haven't said anything to either of the kids then, hopefully, there wouldn't be any awkwardness if the parents said no.
What it comes down to is that you have to do what you feel is best. Each situation is different. I don't think any one of us that have posted are wrong. We just come from different backgrounds and experiences. That is what makes this board so wonderful. You can examine things from all sides and then make a decision. Good Luck.