You are making the classic mistake of asking what can a computer do?
Well it can do anything.
What you need to ask is what does your employer want?
If all he wants is a record of what he has done, any text file generated by any text editor can store that information.
Counting stock he can do with a spread sheet, but at every point of sale, the spreadsheet must be updated.
If he has an EPOS (electronic point of sale machine) this can be connected to a computer database so he can keep a record of everything going through the till. This is probably your best solution.
The hardware is easily available and uses a standard (RS232) output.
MySQL is now an industry standard database with GUI front ends and all sorts of tools for the setting up and running of it.
Everything about database management and setup is in the tutorials from the mysql website. (SQL was developed so managers could use it. That means it was written for idiots).
The tricky bit is getting the database schema correct so it produces meaningful information.
By the way, if he wants something he can show the tax man at the end of the year, he is going to need an accountant.