Are they warming up enough?
An episode with things to include in a run warm up
http://www.mobilitywod.com/2012/03/warm-up-brian-mackenzie-running-warm-up/

It could be a running technique issue.
Are they letting their heel kiss the ground?
If not their calf is contracted the entire time.
It could be a landing error.
If the person is landing on their toes vs landing ball of foot that may be the cause.
A video that addresses this http://www.youtube.com/watch?feature=player_embedded&v=23l42px5R_E