How good a friend is he? A friend and I tend to travel together periodically - I visit him, he visits me, we go somewhere together, he rents a car, I use points on a hotel. We never try to settle anything, just mentally note who's paying for what and next time around the other person steps up. We both assume that, in the long run, we'll do the right things and it will even out.
If you really want to put a $$$ amount on it - half of the $100-$150 range puts it at about $625. Half of the value of the starpoints is about $700 at 2.5 cents, or an estimated $5-600 at the new post 2/1 valuations. It sounds like it's converging on $600'ish