For starters, the floor will need to be cleaned every day. Yes, every day. It's what you can't see that will do more damage than what you CAN see. 2nd, put carpet runners and area rugs down in the high traffic areas. 3rd, at least vacume the rugs up every day. When they start to look worn, clean or replace them. Yes, it will cost your friend some $$$ but he can pay now or pay later when his floors need to be done. Think of the number of hours or days he'll have to close down to have his floor re-done!
Basically, you'd do the same thing at the cafe as you'd do at home. A lot of homeowners LOVE their new hardwood floors, then they cover them up so they don't get ruined. They put area rugs down in the rooms and runners down elsewhere. That's OK w/m (it's just that the beauty is covered up, and the only floor you can see is around the 'perimiters'.) So, just carry the same principle into the cafe, with the addition of the daily mopping and vacuming. Oh, change the water regularly. I'm sure it's not all that long before the clean water is pretty much diluted with rock salt! It's cheap to do so, just do it! A quick sweep of the floor would be good BEFORE the mopping to get the 'big stuff'. If you were to drag around enough 'big stuff' in the mop, it will scratch the finish.
My best to ya and hope this helps.
PS: Are you at liberty to say where the cafe is? I go to Boston enough (since I use to live there from '83-'92.)?