Has your guy...
Whoa! Stop right there! What's with the bleepin' sexism???
I don't have a guy. I AM a guy!
... ever rented a tux onboard? Did it go smoothly or did you have problems?
No, I have never rented a tuxedo onboard. On my first few cruises, I opted for the dark suit then decided that a white dinner jacket would be a better choice for cruising, as most cruises go to warm destinations. I rented dinner jacket outfits from a local tuxedo shop for a couple cruises, then realized that I could buy the basic outfit for the cost of two or three rentals. I subsequently purchased a third shirt for longer cruises and a couple sets of cuff links and studs to provide additional variety. On one cruise, though, I did rent cuff links and studs ($5.00) when I discovered, while unpacking, that I had forgotten them.
Most tuxedo shops ashore will rent a "cruise package" (jacket, pants, two shirts with studs and cuff links, a cummerbund and matching tie, a vest and matching tie, and shoes if you need them) for about ten or twenty dollars less than the cruise lines charge. OTOH, you do have to pack it if you rent it at home.