Masterbation and Penile Numbness

I would think having a sore penis would be a bigger problem than any potential numbness. Masturbating a lot will cause the nerves on your penis to grow temporarily numb though this should go away on it's own within at most an hour or two. If you feel a distinct numbness after having masturbated several times in a limited time period I would suggest that you let your friend rest overnight or at most a day or two. I've never heard of the numbness becoming permanent and I would imagine that you would have to abuse yourself pretty badly for such a thing to occur.

Just keep it in moderation like everything else and you shouldn't suffer any ill effects.

Though if you still feel numb several days after a more excessive session I would strongly urge you to seek consultation with a licensed urologist.