Did your friend take a fall? Possibly a hairline fracture of the tibia or fibula?
Is it a general, ongoing pain? In the muscle? (Cramping, possibly? Mine will when I’m asleep and I stretch my leg.) deeper in the bone? (My guess was a hairline fracture.) In the tendons? (Possibly a pulled muscle?) Could be a number of things. Definitely your friend should consult a doctor if it is unceasing.