I also have both an RB5 and ZX5 and agree fully with all of the previous comments.
I'm new to RC, started about 6 months ago so I can't really say I'm competative but the results for the cars above speak for themselves.
They are very easy to work on and spares are indeed very cheap and easy to come by.
The only negative thing I would say about the RB5 more so than the ZX5 is that there are very few people running them in comparision to some of the other cars. This means that if you're new to the sport and need help tuning your car to a particular track, specific set up advice can be hard to find.
That being said, if you do manage to bump in to someone else who runs one, like Stu Wood for example, then they always seem happy to help out.
Now to the sales pitch.....
I have a spare ZX5 which I'd be happy to sell if you're friend is interested.
Cheers
Martin