Mojave Security Update 2020-001

A friend of mine, who unfortunately is currently living 250 miles away, attempted to install 2020-001. As it said it was going to take 35 minutes, she left home to do some sort of errands. She says that when she came home, the computer had shut itself off, and that there was "some sort of problem". Today, the iMac (late 2015) is "acting up". Unfortunately, that's all I know.

Does anyone have a suggested workflow for getting this system back up and running? Restoring using the last Time Machine backup before the update as a "placeholder" to at least stabilize the system and make it usable? Reinstall Mojave from Recovery partition?

Thanks in advance.