I get calf cramps (and shin cramps too) if my seat is too far forward. The fact that it's only happening at high cadence points to a fit issue, IMO. Has he had a fit done? If so, has he been back to the fitter and mentioned this problem?
What about pedaling toe down? That's really stressful on the calves. Does he try to keep his feet level and drive through his heels? He may need to lower his saddle bit by bit as he learns to level his feet - that's how it worked with me.
Speed comes from what you put behind you. - Judi Ketteler