The only mag that is a 100% sure thing to work in that rifle is a Norinco .223 mag. 30 rounders go for anywhere between $70.00 and $100.00.
While I have a .223 AK variant, I have never been a big fan because there really is no standard. Every country that produced a variant makes them a little different and very little will interchange between models. Even Norinco and Polytech, made in the same country, take different magazines.
From my experience with my BWK-92(Post Ban version of the 84S):
The Pro Mags may fit and function out of the package, but will likely need some file/sanding work to lock in and function correctly.
Bulgarian 5.56 mags may fit and function, but will likely need some file/sanding work to lock in and function correctly.
Another possible option is the Polish Beryl 5.56 mag. I have heard that these will work(probably will need some fitting). I have a few, but have not had a chance to test them out yet.
I think it is admirable that you want to get him another mag(s) for the rifle, but it isn't going to be an inexpensive proposition no matter what mag(with the exception of the Pro Mags) you decide to go with.