+1 what threeputt said.
Got my 1st printer in 2004 (SP-300) and was the very first in my area so many competitors asked me to print for 'em knowing I'm very picky on colors and always deliver great jobs. So I said YES and shortly after, they were rushing me at every jobs claiming they would lost the client if it wasn't installed the next day. Files came un-calibrated to work with my settings so I had to tune colors on almost every jobs. At the end, I was working 16 hours/day, sometimes putting my own jobs on the side. I made 'em look professional because the prints were awesome and they all, one by one, bought their own printer. Sure, they paid whosale prices so I wasn't really making good money and now that they established themselves as dependable sources for large format printing with the jobs I printed for 'em, a part of my own customer base has shifted for my competitors.
Take my words: IF YOU WANNA DO IT, MAKE SURE YOU'LL GET GOOD MONEY OUT OF IT w/o sacrifiying your clients. He'll end up getting his own printer sooner or later.
The only shops I'm still working with are 2 places that make amazing carved signs and really don't wanna invest on a printer, they just have some requests from time to time...