Far be it from me to question a shop mechanic... which I do, and they hate it... But as others have said, personally I'd be looking at just re-building the one wheel with new spokes. I'd really question the "run it's useful life" bit on a rim from a disc brake equipped bike unless there are cracks or other deformations in the rim. The only rims that I've ever had "run it's useful life" and die were either because I pulled a spoke nipple through the rim, or completely wore down the brake track.