Just wanted to toss out a quick caveat to the kenpo recommendation upthread. While I will heartily second them, I'll do so with a couple of areas to watch out for. I have a bad right knee and theses things in particular were problematic for me, not outright show stoppers, but they were difficult.
1. There are a few techniques that require a dropping knee movement. This is faciliteted by moving into a close kneel stance then returning to a neurtal bow stance rather quickly. Twisted Rod is the one that springs immediately to mind but I recall others, just not by name. Depending on how bad you friends knee problems are these will be problematic.
2. There are several chicken kicks in the second kicking set, again depending on the severity of his knee issues this could be a problem.
3. Twist stances were uncomfortable on the knees. As were the afore mentioned close kneel as well as the wide kneel.
4. I recall at least one techniuqe having a jump in the middle of it, as well as many of them containing a lot of stomps. This could be problematic as well.
Given the emphasis that kenpo places on tailoring the art to the individual, should your friend find a good instructor all of these can be worked around depending on the degree of the problems with the knees. These were just areas that I had trouble adapting to after my injury and subsequent surgery, so I though I'd give you a heads up.
Mark