The drives format makes no difference, it's just the file's format that matters. Don't use iTunes for much, but I'll bet even it can read/import just about any music format around (mp3, mp4, aif(f), wav, etc.). Just open the drive, or have him do it (he needs to know this kind of thing, anyway) and have him drag each music file into an open iTunes 'library' folder just for these songs (at least until he knows where he wants to put them). He can see the drive on his DT, double-click to open it, there will probably be a few folders in there. Many of them will be useless, but at least one will have the music files. Command-a to select all of them and drag them into the iTunes library. Done.
No need to reformat the thumb drive, IMHO. It's already read/writable by either a Windows or Mac computer. If he reformats it before retrieving his music, he will probably erase them. If he reformats as a Mac drive, many Windows computers will not be able to use it.