My friend had (moobs),not severe, for 3-4 years, he's 20. he's had doctors look at it and ultrasounds, they found nothing. any explanation? cancer?

Doubt it: Moobs are usually either left over from puberty (50% of men have them for a while), excess fat, or can be triggered by some meds. When they persist after 2-3 yrs, if medical problem has been excluded & weight normalized, your decision comes to either have surgery or not. Moobs that have residual glandular tissue must have it removed surgically & liposuction is done at the same time.