Selecting water pipe for bypass
First off, I'd be discussing this with the plumber, not his wife.
Since the final line being replaced is the cold supply, any one of the three could be used. Since it's being installed in the attic and probably running down walls, I think the PEX is your best bet. You can run it from point to point without any joints. We've all seen PVC leak from time to time and running it in an attic could expose it to cracking if anyone's up there moving around; they'd be less likely to damage the PEX. This might be considered to be an opinion based question so it could get bounced.