If he recently blew his motor up and melted a piston, there is a 99% chance that the oil was contaminated with debris which in all probability made it's way to the chra and has caused damage to the oil seals and quiet possibly the bearings and shaft of the rotating assembly. Please tell me he's not still running the factory oil cooler after this recent melt down? Turbo has got to be checked out if leaking that much, it's hurt. Especially after a melt down like that. One last thing, make sure the oil system is clean and up to par before throwing on a new turbo. Especially a dual ball bearing one as they take much less oil for lubrication and all it would take to toast it would be a clogged oil feed fitting. Depending on how bad the current one is hurt, it might not be a bad idea to break in the new motor with it on the car. Just keep in mind that the longer that worn out leaking turbo is run, the faster the bearings and shaft will start wearing out, and before long, the wheels will start remachining themselves into the housings. We don't want pieces of compressor wheel debris flying into this brand new motor.
Just my O2s worth.
Patrick