The Sony HVL-42 is the medium flash, HVL-20 the small one that is better than just having the camera's own flash, and HVL-58 the big one that does everything.
I have the older Minolta 2500D, 3600HSD and 5600HSD (Smallest to biggest).
It all depends what he wants to do. I find I use the smallest, least sophisticated one the most because it's easier to carry around (good for parties in a small place). But the big one really shines at wedding receptions and other big events, and for when one want to do all the other really neat flash stuff one reads about (the medium HVL-42 can do that too, but the 3600HSD wasn't as sophisticated).
Sigma and Metz also make sophisticated flashes for KM/Sony, the Metz generally being dearer, and Sigma cheaper (but still pretty good). But beware if hunting for these on Ebay or other internet sites. If they say they are for Minolta, they will not work on KM or Sony. You have to find them saying they're for Sony.
Renato