Hey there im not too sure how to answer but, what i do know is that any time plates that have to be fastened to the bone with skrew's or bolts you put a lot of stress on the fasteners (or more importantly the surrounding bone), the plates are strong but the bones are not in comparison. bones are complex, they are strongest in compression and weakest in bending. bending (a blow perpendicular to the axis of the bone) is usually always what causes a break (or clean break) spiral fractures are a combination of bending/compressive impact. anyway steel is twice as strong in bending than bone so a future blow to that area would transmit to the fasteners (or more importantly to the underlying bone). on the other hand usually the extra strength in the area will prevent a break in that region... another thing to consider is as bones get older they become more brittle and this may cause problems at the fasteners. anyway its not a new thing there is a lot of knowhow in bracing bone so its not likely that anything will go wrong. im not too familiar with bone graphs but iv also heard my fair share of negatives about it. in the end a consistent structure is healthiest and natural, something that bone graphing may provide... but get him to ask the doctor and as was suggested to me on this forum with my broken collar bone, make sure he makes it clear to the doctor that he plans on wake boarding in the future, the doctor will have more information and will proceed accordingly.
its interesting that you pointed out that he was wearing a knee brace, i understand that he broke his tibia and fibula, if the blow was to his knee and his tibia-fibula broke then its obvious that the blow was transmitted from the knee to his lower leg. remember energy doesnt disappear, what the knee brace does is transmit the energy away and hopefully over a greater area. that in combination with an impact or twist to the shine is a typical outcome. don't blame the brace though, id hate to see what damage would have been done to the knee if he wasn't wearing one. In anatomical terms a break away from the joint is always better (more manageable) your knee is a very important and complex joint.
i know i didn't answer your question but i hope i gave you (or him) a bit of insight into whats going on, in my own opinion he was lucky that his knee didnt explode.
this is what a fibula plate looks like: