Personally, sights and furniture are the biggest deals for me.
First off, for me the LOP of the standard stock is way too short with the way I like to assume an aggressive stance and lean into my rifles. I am 6'5" and with the standard stock, the rear of the receiver thumps me in the face with every shot. I have an ACE folder on mine right now. The ACE folders are more expensive than the TAPCO folders, but are much more durable--and thus--more functional. In addition, the wood furniture that came with my WASR had a steel pin through the front of it that would warm up very fast and burn the shooter. I replaced these with TDI synthetics, which had the added benefit of allowing me to experiment with a VFG. I find the VFG affords me more control and keeps my hand cooler. The set up I have is very solid and works very well for me, but there are tons of options out there.
Secondly is sights. I think the sights are the weak point of the AK. At the very least I would recommend a set of Mojo ghost ring sights. They run about $40 and although they can't extend the AK's short sight radius, the drastically improve the sight picture. Most AKs, or at least a lot of them, have a siderail mount for optics. This makes it very easy to mount a quality Russian optic like the Kobra, PK-AS, or the fixed magnification POSP. Adding such a sight will make you quicker and more accurate and I highly recommend it.
Thirdly, possibly, would be the trigger. Some AK's have really bad trigger slap. Don't ask--you'll know what it is if you have it. If your finger is numb after a full mag, you need to invest in a TAPCO G2 trigger group. They aren't terribly expensive $50 to 60 with a detent plate--and they drop right in if you're willing to take 30 to 45 minutes of quite time to learn how to do it.
My recommendation would be first of all just to have him put at least a couple hundred rounds through it. Have him shoot it from the bench, then take some snap shots offhand. Once he puts it through a variety of conditions, he'll have a better idea of what he likes and doesn't like, and that will give him a much better idea of what he, personally, should replace.
