I wonder if he is blowing smoke?? If the rear main seal leaks enough that you lose oil pressure to the cam shaft (enough to cause harm) you would be pouring ALOT of oil into that engine on a daily basis. You should be able to see some oil dripping somewhere if the rear main leaks.
If you replace it yourself figure at least 18 hours (I work pretty slow). Seal is cheap and easy to replace once you get to it
. Replace the front seal on the tranny while you are in there (if it is an automatic).