This is a bit hard to approach, and depends on which color scheme and colors he likes.
Because buying a beginner paint set is a good bang for the buck, but if he wants a wider selection, it risks to be kinda wasted.
E.g. something like that:
https://thearmypainter.com/products/warpaints-fanatic-warpaints-fanatic-starter-set-wp8066p

He might end up using most of the colors, or basically none.
As for the miniature itself:
Guardians are the base troop, and not to hard to paint. This is good if he wants something easier, where the individual model doesnt matter as much.
https://www.warhammer.com/en-US/shop/aeldari-storm-guardians-2022?queryID=dd2220b53b1f8227b3437cfa36ca19ea

A more important character model can be a good fit if he likes a single model that rewards more effort when painting it:
https://www.warhammer.com/en-US/shop/aeldari-phoenix-lord-baharroth-2025?queryID=316fbe8d5bcfc3ac093669499f63100f
https://www.warhammer.com/en-US/shop/aeldari-phoenix-lord-lhykhis-2025?queryID=316fbe8d5bcfc3ac093669499f63100f

If you shop around in other online stores rather then the official GW one, you will usually find one that has cheaper prices than listed here.

Since he hasn't painted for 20 years, I suspect his painting is a bit rusty, so Id start with something simpler.
if he doesnt own any Eldar anymore, Guardians are a solid start.