If only one or two lines are stained, I would just regrout. It's quick and inexpensive. If not, why not try to sell the client on cleaning and color sealing. If the floor had been color sealed, the spill would have been easily wiped away without a trace.
To go along with my color seal demo kit, I've included a small "floor" (four 6" tiles; sanded grout) that has been color sealed. I plan to have potential clients spill whatever they want on the floor and then wipe it up so they can see for themsleves one of the primary benefits of color sealing.