Sorry to hear about your customer / experence. Been there done that (in my former job - furniture delivery trucks, expensive homes = *ssholes) As posted, brake cleaner or starting fluid and cement rubbed in after if the drive wasn't sealed. Be sure to have rags handy at spraying, don't let the cleaner solution flow around or the spot could leave a lighter larger discolored area. Ring the area with rags first, spray and wipe up. May take a couple shots, depending on how long it's been there. In worst cases (unsealed concrete, the cement powder seems to color it back to normal color . . . .

As it crosses the mind - due unto others as they do to you, I tend to, live and let learn. Two bad's don't make a good . . . .