Pony panels is really a brand specific terminology from Custom. However their pony panels are a lot of brands standard panels. For Custom it means theoretically shorter and less deep than standard. However I have always found the Custom standard panels longer and deeper than most brands standards panels. Therefor a lot of other brands standard panels are the same depth/length of the Custom pony panels.
For example, Standard panels on a Black Country are typically shorter and less deep then even Custom’s pony panels,
There are a lot of panel configurations and if you are shopping used, you just have to look at how the panels tie into the tree. Plus it gets really tricky if your friend needs a 17.5" seat and the pony can only take a 16.5" tree.
It would really help to know what saddle they are currently riding in to make some recommendations and of course pictures would be even better.
I hope this helps and good luck!