Whatever you end up doing, I would recommend that you address the issue up front before you commit to the trip. Otherwise, there will be hard feelings: either your own for feeling like you should have gotten something for your hard earned miles, or your friend for feeling that you "cashed out" on his back.

I know if I were sharing trip costs I would feel an obligation to make good somehow to the person who is cashing his in.

Does your friend accrue points and miles? Does he value them himself.

I might pose it this way: "Listen either we can both pay cash or I can use some points. But if we do use my points, I would like to be able to share at least some of the value with you. Otherwise, I should probably wait and use them for a speical occasion"

And bottom line, if your friend wants to use your points and disregards the value you place in them, you have to decide whether it's worth losing the friendship over the points.... of course you can withdraw the offer to use the points.

Rita