The best tuxedo on earth won't impress unless the fit is right.
When arms hang down, sleeve hems should end at the base of the thumb. The cuff of the shirt should just peek out from the sleeve of the jacket.
The jacket should fit smoothly across the shoulders—with no pulls when arms move or loose gaps.
Slacks should break slightly at the top of the shoe. Wearing dress shoes—he can borrow some from the rental shop—when trying on the slacks will ensure they are hemmed to the perfect length.