My friend has a pain inside of his left calf (not charlie horse)and his skin behind his knee has burning sensation

Curious,

If there is any redness, swelling, or history of injury, it would make it especially important for your friend to make sure there is no blood clot causing the pain. Rather than take any chance at all, please ask your friend to have the doctor examine his leg. Other possibilities might include some sort of cyst or other growth causing the pain, arthritis, or just a simple muscle pull, or even nerve damage. Since we don't have any other information about your friend, such as medical history, activity level, etc. the best thing to do is obtain a diagnosis from a healthcare provider. An examination is the best way to pinpoint the exact cause. It is always better not to delay, because the earlier something wrong is discovered, the greater the chances of sucessfully intervening.

Kathleen