Or just get another separate external storage device and move them on to it and get one with the power switch that you can just turn off when not needed. It certainly wouldn't be available for access if it was powered off when not needed.
This is the best solution of the mix. Get a USB drive, put the "special" stuff on it and once on, eject and unplug it from the Mac. Teach her how to do that correctly. (Eject first, then unplug.) Then put the drive somewhere she feels"safe"about. When she wants/needs the files, plug it in and it should mount and let her get to the files.
Turning on her firewall is a bit of overkill, too. Unless she has a portable and takes it to use on public local network like coffee shops, etc., the probability that someone will try to get into her Mac directly is very low, virtually zero. But if it makes her "feel" better, it probably won't do much harm to have it on.
She really needs to be educated about the real risks, as Patrick has indicated. What you see in movies/TV and read in the mainstream press is so over the top about security risks that people over-react and lost the ability to think rationally about it.
For example:
1. Her internet access modem/router probably has a robust firewall (99.9% do). That should prevent most attacks from the Internet into her own local network.
2. Her WiFi connects to a local network with local network addresses, not directly to the internet. Even if she has just ONE device using the network, it is a fully separate local area network on the customer side of the ISP router. All connections go through the router, which has a different address within the Internet Service Provider's own network, and (maybe) that eventually has a port to the real Internet. Or there may be another layer after that. The bottom line is the "real" Internet is layers and layers away.
3. Nobody can connect to her local network without the password for that network, and if she has a robust one, that provides a LOT of protection.
4. Nobody can connect to her Mac unless she authorizes it, even if they somehow manage to get inside her local network, or the ISPs network.
5. 99% of data theft from individual computers come through human failings--falling for click bait emails, installing apps from places that aren't trusworthy, installing pirated software with malware in it (you get what you pay for), falling for phishing emails/texts, etc.
Typically, naive users like this one get taken by the bad guys just because they don't know how things work. They install "protection" that is actually malware that gives the bad guys access, when if they had simply done nothing, they would have been secure.
What she is doing (turning off the WiFi, wanting a separate computer, etc.) is a bit like taking the wheels off your car each night after you park it in your garage, just in case some car thief migt break into your garage AND has the keys to the car. Over the top.
