If he is using gear oil in his transmission, give the leaking oil a good sniff.
You'll be able to tell if it's coming from the inner or outer by the smell.

 

A leak from the outer primary will be continuous as will transmission leaks until the fluid level drops to below the mainshaft. A leak from the inner primary will leak after a ride for a while and then stop; this leak will leave a spot the size of a half dollar or quarter before stopping.

It is unusual for the outer primarg gasket or drain plug to suddenly start leaking; just doesn't happen. Those gaskets are intened to go on dry; Yamabond, silicone sealant, shellac, etc. are not required. You should consider expanding the search for the source of the leak as has been suggested.

Time to pull the inner primary and check for leaks; check the shifter shaft seal, mainshaft seal, quad ring and mainshaft/5th gear seal. It is very common on the early models for the inner primary bearing to "walk" in on the mainshaft/5th gear seal causing a leak. If you replace one seal behind the inner primary; replace them all. If you don't find anything while in there, replace the IPB seal in the inner primary housing. I will be a six pack you find the leak.

 

You can buy a new inner and outer primary cover for about $500 if all else fails.

 

I would bet shifter shaft leak, unfortunately not an easy fix for a 35 cent seal........

 

Another thing to consider the bike is 8 years old, and without knowing the life of the bike (IE: over torqued bolts/plug, methods used in cleaning mating surfaces, and the extremity of hot/cold cycles, not to mention if something ever grenaded inside that may have created a stress fracture.) I would say its safe to figure on taking it all down and replace the seals and have the mating surfaces checked. Doesnt the 05's still use the round gasket in the center of the cover at the inspection cover bolts? If so this cold be where it maybe coming from.

 

+1 . This was the problem I had with my '09 SG. Drove me batty until I took it to my Indy. Was primary out put main bearing.

 

If someone overtightened the drain plug, the primary cover may be cracked. Good used ones can be had on eBay for a few bucks. That would be my next move given the information...

 

+1 on shifter shaft seal. That is a common problem.