Rich could mean too much fuel, or too little air. Lean is the opposite.
Usually the phrase "new exhaust" means an aftermarket muffler or complete system which allows more flow than the stock unit, thus making the mixture leaner unless the carbs or injection have been recalibrated to compensate.
So it sounds to me like the bike is running poorly and the owner is just taking an uneducated guess as to what the problem is. Or just plain BS'ing you.