Depends what kind of work you can do.
The best would be to extend the ethernet (from the existing router) or even the ADSL/CABLE (source of internet) up through the floor, and into another router of the second floor and have a seperate network for that floor. However, this would involve a lot of work, and if extending the ADSL/CABLE, an electrician. The cheaper option would be to place a wifi repeated/extender directly a floor above the 1st floor router, and then have more on each ends of the building. However, wifi extenders cut the speed a bit, but not too too bad (better than nothing
.
Also, there are these things
P) called power Ethernet adapters. What they do, is they transmit an internet signal through the power in the building, recieved by the adapter on the other end. All you would have to do is run an ethernet cord from the existing router to one of these things (plugged into a socket) and run another ethernet cord from the other one plugged into the wall on the other floor. From there, your Dad could plug the ethernet into a modem (at his desk), provide wifi to the rest of the workers around him, and he could still have another cable running from the modem directly into his computer, giving him great speeds. Usually, these are a very good options, however, the more power consuming items plugged in, the more "noise", and thus a slower connection. I still think this is your best option (in terms of value and speed)
The best options would be the get an ADSL/CABLE connection directly to the 2nd floor, but if that is not possible (as it is a big and expensive job), I would get the power line Ethernet adapters.