People, why discourage him?
The 500 Plus is a great computer, the first amiga i ever owned, and it is very upgradable.
First of all, remember that ACA520 are coming, and they should offer a passthrough for A1200 CPU expansion. Imagine an A500 Plus with a Blizzard PPC board (and there is room for it the large A500 case).
Then, there i a lot of expansions for the A500 side slots, either IDE or SCSI controllers, RAM expansions, etc, that usually sell rather cheap compared to other amiga hardware and are not rare. A590 is a good choice and of course works with the 500 Plus.
He can also use a zorro adapter for the side slot and use anything from RTG boards to a Deneb.
And finally, the A500 Plus can be fitted with the Indivision ECS, which is not only cheaper than its AGA counterpart but also a lot more powerful and can almost become a mini RTG board.