That machine will have USB3. So take a USB3 stick and do a macOS install on that. Then boot from the stick. Hold down the option key when powering up and you'll see boot options. Select the USB stick. It'll be slow but at least you can test apps.
The faster the USB stick the better. Or better yet get an external NVMe drive with USB connection. I've got a couple. I've got an older one with 10.13 that I've used for testing a lot - I boot various Macs from it. It runs surprisingly well.
What we don't know yet is if you're dealing with a hardware or a software problem for sure. It's good that it passed diagnostics but those diags are pretty superficial.
I've been thinking another good test would be to replace the internal drive with a new one. Install a clean Mac OS. See how it runs. If it's smooth and reliable then you can reinstall her software and she'd be happy.
These kits come with a new internal NVMe drive.
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The idea is you put the old drive in the enclosure they provide. Now your old drive becomes an external. Well, in your case maybe put the new drive in the enclosure, install a clean Mac OS to it. Boot from it. If it runs well you've likely proven the hardware is OK before opening up the machine and moving stuff around.
Virtualizing on her new MacBook might be a solution too, but that depends on whether her MacBook is Intel or Apple silicon. If Intel she could run High Sierra in a VM quite nicely.
However, I suspect she likes the Mac Pro - big screen and other stuff connected to it, etc.