Traveling with friends I figure it all evens out in the long run. Example, went to a hockey game in Denver with a friend. I picked up the flight with miles, he picked up the room with points. Close enough to even not to worry about it.
I'm also taking a couple friends to Europe in the fall. I had enough AA miles from two credit card signups for 40K awards, but had to do an open jaw with a cheap one way in the middle. I made them an offer up front - I would get the flights and hotels I could with points. They would pay their airfare taxes & one way ticket, rental car, gas and the two hotel nights I couldn't get free. I estimated their travel expense would be about $500 each. It is a win-win since they get a trip to Europe for $500 and I save on the charges I would not be able to cover with points. In effect I sold them 72K AA miles (after CC 10% rebate) for about $700.
Whatever you do, discuss it up front so there is no miscommunication or hard feelings.