Q.Should you prune ornamental fig tree back to its roots?
We have an ornamental fig tree in our indoor garden, and it’s been there since I was a little girl. I have pruned it back a few times, but my brother has recently taken over care of it and decided to prune it down to the stem. I don’t know if the intention was to begin removing it, but he’s taken a healthy, 20-something foot fig tree and sawed it back to the stem (like you would a Christmas tree). First of all, my question is, in what cases could you do such drastic pruning? Secondly, is there any hope that this tree will recover?

This is not an ideal pruning job on your Fig Tree, but Fig's can be quite forgiving and can even die back to the ground in cold climates and come back healthier than ever.
Give it some time and it is likely the roots will began to start sending up shoots.
Here is a link the discussed pruning for Fig Trees.
https://www.gardeningknowhow.com/edible/fruits/figs/pruning-fig-trees.htm