my neighbor has a sportster('08) with a leaky exhaust gasket. he has stock exhaust and is too cheap to buy louder mufflers...he likes the extra noise from the leak! my question is can this leak cause long term problems-such as burning a valve?

 

if it's leaking bad enough at the exhaust port on the head and you're sucking in cold air, it can certainly warp a valve but the odds are very slim that would happen. One cylinder has back pressure and the other doesn't. either way, it should be fixed no matter how much he "likes the extra noise"

if he likes extra noise, tell him to grab a set of woods cams and install those, at least it will improve the performance of the bike. :-)