using CF as an injury prevention program would be a good idea for him, at the least. things like air squats and back squats would help alleviate tight hamstrings (he's a runner, i'm sure he's got 'em) and balance out musculature between antagonistic muscle groups. deadlifts, OHS, and other back-centric exercises help with posture. etc.
that being said, you can always pull out the argument that crossfitters are hotter than marathoners. then you stick out your tongue at him. :harhar00: