There wasn’t a wr450 model until 2003. Up till then it was a wr426/400. If he doesn’t have a manual so go to the following link and download the appropriate manual for his bike. There should be instructions in it regarding oil change procedure and oil quantities.
http://www.yamahaownershandbook.com.au
My 04 wr450 takes about 1140ml, but I always check it on the dipstick after warming up the engine for a few minutes, then stopping it and letting it stand for a minute before checking it on the dipstick attached to the small rubber cap. It is important to follow the procedure as above and don’t check it on the dipstick when the engine is cold, or you won’t see anything, hence giving the impression there is no oil in the system.
NOTE the dipstick is on the LHS of the steering head in front of the tank. Don’t get confused thinking it is on the oil filler cap on the LHS of the crankcase.
The following image isn’t too clear, but it is part #35