Mac Lightroom updates cause Presets to disappear

Probably Presets stored with the catalog rather than the system. Many people keeppresets stored with the catalog, however it is advisable to always make a copy of the presets file so that when there is a new Lightroom update, you can restore the presets where you want them.

Yet another possibility (but do try the first two mentioned):
There are some presets which are only partially compatible with a selected photo as not all of the settings of the preset can be applied to that image. If you want to see the partially compatible presets, go to Edit > Preferences > Presets (Windows) or Lightroom Classic > Preference > Presets (macOS) and select Show Partially Compatible Develop Presets. Better?

Another possibility -that several people have encountered and confirmed- is that she used to have the presets stored with the catalog, and now this got reset so Lightroom is looking for them in the default location again.

We'd need much more detail to figure out why. Encourage her to post directly here.

One guess is that somehow she's installing them in the system folder where LR stores its built-in presets.  There's lots of misinformation out there about how to install presets, leading people to try to manually place them in the wrong folders.  The easiest and most reliable way is to use File > Import Develop Profiles And Presets.