Copper for anything in a vehicle is a bad choice.

It's soft & it will fracture wit vibation.

Think about this.

Why aren't brakes lines, gas lines, and coolant lines made from copper
from the factory?

Bceause it'll crack, and it's an unsafe selection .

Why didn't your mechanic make that water pump to oil cooler pipe out of
steel line as it was ??

This would have been a much better selection of material.

I'd have him take it out and start over.