I run stock line with zero issues. Over 45k miles. I have the airtec exhaust manifold.
The inlet oil pressure to the turbo isn't very high. It's from the head so it's not even near as high as the pressure at the oil pump, then further restricted by the line. That's a pretty hair brained theory to me.
I would ask him if he has disassembled the turbo and saw the telltale signs of oil starvation. Any competent mechanic should be able to identify oil starvation/overheat, contamination, foreign objects, etc. A turbo, a cylinder head, it all looks.thr same.
If you don't have to run an aftermarket line, I would always avoid it. The line, it's size, it's location, and any orifice features were engineered to provide proper inlet pressure range for a bw journal bearing system. An aftermarket line is just whatever -3 or -4 line they have sitting around cut to length, with universal fittings crimped on....