helping somebody on a B3 that keep pushing drive lube in thru transom assy, been resealed 5 times he says, I was unaware of any way this could happen other than the reservoir line ?

Pinion seal leaking more than likely, filling up the bellow and running into the bilge, pull drive and see if bellow is full of oil.

what could cause this to fail 5 times ?

The only way to determine the pinion seal leaking is to pull the drive. If it is leaking the input shaft/yoke assy/pinion gear assy has to removed from the drive and taken apart to replace seal. Has he done this 5 times or just put some sort of sealer around the out side of the seal an hoped for the best?

No, a merc dealer who I know well has done it 5 times, now only last 20-30 mins everybody is scratching their heads

May have a bad pinion bearing causing yoke to move screwing up seal. Also if the yoke was not shimmed right, it could be to close to or rubbing the seal and burning it up.

Have you found any oil in the bellow/boot to see if the pinion is leaking?

If the seal surface is damaged it won't ever seal. You can install a sleeve on the yoke to repair that area. If the seal is moving in the retainer it can be staked or if it is an XR type case there is a new retainer that has a lip on it.
Hope that helps.
Dick