If you do the deal, make sure you get your TOTAL COSTS of a new XD covered. (If they sell in your area for $450, get $500 to cover tax, paperwork, etc.) No need to try and 'get over' on a buddy but doing him the favor shouldn't end up costing you anything either. An HS2000 is not a collectors piece. It's not like 1911Tuner losing a gun that had a ton of style, probably some sentimental value and, if not now, maybe eventually, some "collectability".
As long as he is what you say he is, don't read too much into it. Look at it in basic terms: Would you trade a used HS2000 for a brand new XD9? I would.