Networking HELP HELP HELP!

Okay so my dad wants me to figure this out :stars:
he wants a computer at home that can run this billing program called medisoft. and he wants to be able to have his secretary look at the files and use the program from the office, so he was thinking of using a external harddrive to take a file, but then i thought the software probably wont allow two computers to use the same cd key, all this needs to be legit so i was thinking what are my options

what is that way where a computer at my house has somesort of windows that when it turns on you just are like connected to a windows on a server, would that work? that way both computers can just connect to one server and then it would be like being on the same computer