Wow the GTX 280, that's brought a whole ton of memories of me being a 12 year old and just getting into the whole IT-thing back!
Yeah the 280 would be a great card to throw into an HTPC, maybe a little overkill, but that would easily run a 50" 1080p TV with no problems. You'd need a better PSU if you had one of those one though, I'd say 650W or 700W. Those things were power hungry, and you'd need a good case as they ran hot too.
Still think a GT 440 would be all he would need though, a GTX 280 would be good, but a bit over the top I think.
Pairing a GTX 280 with a Celeron would just be weird as well!