By "get a recorded track to..." I take it you want to be able to Export (Share) a mono audiofile from GB? Unfortunately, AFAICT GarageBand doesn't offer a simple way to do this (apparently there may have been a mono option in the past, but it's not there in the current version). Logic also makes it kind of a pain to export in mono, but there are options for it there—I can't find any of them in GB. The best thing I can suggest is to display the Master track (Track menu > Show Master Track) and do one of the following:
1) Insert a Direction mixer in the plug-in panel (in the Smart Controls pane). There you can set Spread to 0.00, which will make the stereo audio mono
2) Insert a Gain plug-in in the plug-in panel (in the Smart Controls pane), and select the Stereo->Mono preset
Either way when you Export/Share you'll get a stereo audiofile of mono audio—the same signal on both sides of the stereo audiofile—your guy should be able to either use that as mono audio (which it is effectively) or split off one channel if he really needs an actual mono audiofile.
That said, to be honest a mixer/masterer who doesn't know how or doesn't have suitable software to turn a stereo file into a mono version seems a bit suspect to me, unless you're providing him with many many separate files and he doesn't want to spend the extra time..? Why do the tracks have to be mono? I get stereo tracks/stems to mix all the time, and while I might prefer any mono tracks to be mono audiofiles, working with stereo audiofiles of mono signals is no problem..
If you really need to convert a stereo audiofile to mono yourself, I would think you should be able to do that easily enough in Audacity (a free audio editor)...or you could just bite the bullet ($) and upgrade to Logic..