Yep, all the above info is correct. A good pro player on any horn will accomodate any key, but in general they prefer keys without too many sharps or flats, and prefer the flat keys to the sharp keys.
Playing in keys they prefer will make them more comfortable and play more fluidly, and also generally they'll be more in tune. The intonation is helped by how much tubing you have to use - on a trumpet you play a Bb or an F (in concert pitch) with no valves pushed down, but a low B needs all three valves down. Each valve you push down adds more margin for error unless each one is tuned perfectly, and even then there's some unpredictability with how they interact. The 3rd valve on trumpets, for example, has it's own tuning slide that they can push in or out with their pinky to compensate - but it's not an exact science and if they can't hear themselves well it's tough to get it right.
When you first start off in elementary school they'll have you playing in Bb or F, expanding to Eb and C, and so on from there. I was a competent high-school and low level college trombone player, and still very rarely saw key signatures even as sharp as G Major all through high school.
High school string orchesta players are the opposite, BTW.. they tend to start off in C, G, D, etc, and are less used to flat keys.
So in short, it depends on what level of horn player and what you want them to play. If you want good solos out of a mid-range player, tuning down is a great idea. If it's just background hits from a pro, don't bother. They tend to learn all keys just to accomodate us rockers.
About transposing as touched on above: Any competent jamming horn player will know how to transpose for their own instrument on the spot, but it doesn't hurt to have an understanding of it: Trumpets & Tenor Sax are Bb instruments - when they're playing what they consider a C, it sounds like a Bb. Alto Sax and I believe Soprano Sax, and Euphonium (not that you're likely to jam with one of those! and also Clarinets, I think) are Eb - when they're playing what they consider an Eb it'll sound like a C. French Horns (just in case you ever cover Quadrophenia) are in F, I believe.
Ok, this post has been brought to you by too much caffeine and too much education. Damn, now I'm in the mood to pick up a trumpet...
I'd just ask the guy straight up what keys he's comfortable playing in. Might be tough to get a really good Jerry tone tuned down but it could be worth it. Or use a capo and go up to F. Either way, all the C songs will be tough unless you can leave them in C. Both C# and B are horrible horn keys.