I agree a Jerry Freeman tweaked Generation whistle in D would be a good bet. It’s a solid instrument, for not a lot of money, but it still has the character of a traditional whistle. It’s predictable, stable, and responsive.
You can email Jerry from his profile page: http://chiffboard.mati.ca/profile.php?mode=viewprofile&u=1845
You can also get Tweaked Generations for $40 or so from The Whistle Shop:
http://www.thewhistleshop.com/catalog/whistles/inexpensive/Generations/tweaked.htm
“Tweaked” means modified to be better than the stock instrument. Some minor adjustments can be done by just about anyone (although you’ll need several cheap instruments to experiment on and lots of time to search this board for tips). More extensive modifications like those done by Jerry Freeman take a lot of experience and skill to accomplish.
I’ve played several instruments (brass, percussion, concertina) and many whistles, and I agree if he’s an accomplished musician, a cheap instrument will leave him cold. I’d avoid the cheapies, like a Sweetone (even a tweaked one), a stock Generation, Feadog, Oak, Clare, and Walton, except to satisfy curiosity maybe (since they are only a few bucks). While they have their own charm and folky character, they’ll probably sound thin and cheap to someone used to professional instruments.
A tweaked Shaw is a good traditional sounding instrument, but it’s a bit rougher and can be tempermental, it’s not very durable. A Water Weasel is a quality instrument with good tuning and responsiveness for $70, but it’s a bit plasticy in feel and sound. The Susato is inexpensive and well tuned, but is even more plasticy – while some pros use them, personally, I don’t like them.
I’ve only tried the “Whitecap” from Mack Hoover, but it’s very good…a brass whistle might be an even better bet, at about $55, and you can tell him to make it a bit louder than usual, his standard instruments are quiet and a bit touchy to play.
Another thing to consider is that you can sell most of these whistles to people on the board here for only a small loss if you don’t like them.