I currently have a 7" Zig-Zag and an 8.5" Tire-track together in a 55 gal. I know that eventually (probably sooner than later with the way this guy eats and grows) I'll be placing the Zig-Zag back in my community tank with the angels, Rams, and such, but as they are close to the same size right now they actuallyu seem to enjoy each other's company. They're almost always around the same part of their tank, but they seem to inhabit different haunts. The ZZ likes to bury himself in the sand with his head sticking out, while the TT tends to hang out in the plants more. But at feeding time, they do this odd sort of "swirl-dance" as they take bloodworms and mysis. I've never seen any aggression between the two, and all fins look perfect.
P.S. - I'm also interested in hearing what the size of your friend's tank is. 'Cause that would make one hell of a show tank!