Unfortunately the only way for your friend to find out what is going on is for your firend to go see a doctor. Maybe you can tell him there is no room for embarassment.
Would he rather be embarassed or know if there is anything wrong with his penis. If he does not seek care, he will not get better, that is reality.He could be dealing with an infection, circulation issues, or even an allergy, etc. Etc. So please encourage him to get a check up right away.