I'd imagine with it working when last shut down due to being put out of service, that some capacitor(s) in the PSU have shelf rotted. Just to rule it out, I'd also check the voltage of the CMOS battery, once in a blue moon I find systems that won't post without a viable battery but it may be premature to even mention this pending further info about "doesn't work" as mentioned below.
With very little load like unplugged from the system, I'd first just short the PS-on to Gnd with a wire or paperclip to see if it stays running (though this is not a definitive test with some OEM PSU which are designed to eek out a tiny bit more efficiency by not incorporating any rail load resistors in them, depending on being connected to the motherboard for the load), and if possible (may be hard on a typical OEM PSU casing) shine a flashlight in and look for swollen or burst capacitors. A PSU dying from bad capacitors can first run on it's own but once you put enough load on it, then it shuts itself off. If you can't see inside, pop the PSU open and inspect caps and blow the rest of the dust out.
Define "doesn't work". Forget about anything plugged into it, what exactly happens when you try to turn it on? Fans come on and stay on, or just try to spin up for a moment, or ???
If she was going to be content using an old HP, why even consider the $1000 build? It seems strange to me that you'd contrast these two extremes when there is a wide gulf in the middle, full of plenty of fish.
Just get her a lightly used OEM refurb for $150-300 or so, upgrading memory and SSD if needed, which it may not need. You didn't mention much about the situation, just that it used to be used at a business but is she now PC-less (possibly using a laptop?) and wants this (or something) as a primary use system?
Unless someone took the sticker off, it probably has a label stating what model system it is which should narrow down the build inside, how viable the components are for XYZ purpose, and possibly even help identify if, and "IF" you want to build with new components from scratch, whether the case is ATX or proprietary. I've put more new mATX builds into repurposed OEM cases than I can count, if they don't need certain things like full length video cards or enough airflow to handle those. You do have to mind the motherboard standoffs available, sometimes rip one out with pliers, or sometimes throw down an adhesive backed rubber bumper support to keep the board from flexing too much. I take it on a case (and motherboard) by case basis.
You can get a heck of a lot of multimeter these days for dirt cheap, if you don't need high precision or high protection from impact/drop damage. There are so many cheap options today ($20 or less) that I don't even know where to start, to recommend one, possibly watching some youtube videos might help.