Assuming that your budgetary cap is a firm, here's the analysis:
First question to ask: which brands are supported by local dealers. It will probably come down to Stihl or Husqvarna. Go with the brand that has the better local dealer support.
Second: what is he cutting? This may be a moot point since you have a firm $400 price limit, but if he was cutting huge logs that would suggest one class of saws and if he was cutting lots of little stuff, another class of saw would be suggested.
Anyway, your best bets for a $400 saw for a homeowner woodcutter would be:
Stihl - MS270/280, or MS290/310. I'd probably go with the 280, since it has a fair amount of power for its size, good anti-vibration, and a magnesium crakcase. The 270 is its smaller cousin (engine displacement-wise, not physical size) that comes in at a few bucks less. Another new member here just bought a 270 for firewood duties and has been very pleased with it. The 290/310 saws are bigger, heavier, have plastic crankcases, have relatively poor anti-vibration and are a bit underpowered for their weight. That said, they are dead reliable and make a fine choice for the occasional saw user who cuts firewood for himself, clears up after storms, helps out neighbors. This is what I used mine for and I was never disappointed with my 290 in the years I owned it; I ran it with a 20" bar and .325" pitch chain. I would recommend the 290 as a great value in a durable saw, without hesitation.
Husqvarna - 350 (plastic crankcase), 353 (magnesium crankcase) or 359 (magnesium crakcase) would be your best choices, with a slight edge being given to the pro-grade construction of the 353 and 359. Lots of guys have been really pleased with the 350, though, and it is a very good value. The 455 Rancher or 460 would be OK, too, they're the analogues to Stihl's 290/310: bigger than they need to be, heavier than they need to be, but reliable and durable.
Any of these saws would be capable of handling a 16"-20" bar. If you're unsure what he would want, get the 16" since it is lighter and more nimble, and he can get a longer bar later if he needs one.
Let us know what you settle on!