I have seen 5 gal heaters that go under the sink. They continiously heat the water just like the main one for the house. You will need a an electic outlet.

are these expensive on the electric side. Can they be shut off for the 3 days the business is closed? Does it cost a massive amount when you turn it back on?

Expensive to operate? Au contraire! In general, these are called tank-less water heaters, and because there is no tank to have to keep water hot all the time, this means it consumes electricity/gas ONLY when it's used. In fact I believe the government will give u a tax rebate because these devices are energy-efficient. HOWEVER, the initial cost maybe more than a regular water heater if u can believe that, but in the long term, definitely cheaper to operate. Your friends better stays there for a few years to recoup or is the owner footing the bill? Heat comes on immediately, waiting time is not an issue. The downside with tank-less is they give u less galon per minute, as compared to a tanked heater, but in a saloon, that should not be an issue.

A tankless electric heater will require extensive upgrade to the electrical panel to handle the load. A cheaper alternative would be a small tank heater. My friends use one in a summer home that they rent. It gets turned off when no one is there. It requires some time to reheat have being turned off, but for what you're doing it would be no big deal. They use a 20gal tank in a closet. You might want to look for something a bit smaller for both the hair sink and lavatory.