GIMP may run on older machines, and AFAIK, can read PS file formats. Plus it’s free. The downside would be a somewhat different interface, and I’m not how well it exports to PS formats.
Try Recycled Software. I didn’t see Photoshop in their list, by you can contact them directly and double check. And there might be something else suitable there. All the software there is legit.
If you buy the latest version but then run an older version, I certainly wouldn’t have an ethical problem with it and I doubt many software publishers would either.
The licenses for Microsoft Office and Windows XP Pro allow you do exactly that I used to purchase computers with Windows XP licenses but Windows 2000 pre-installed.
Has she tried some of the local schools? I am amazed at some of the ancient software that is always showing up in the staff lounge’s “Free to a Good Home” box.