Hello
thanks for wanting to help out your friend.
Masturbation comes on as a result of emotional and
physiological changes occurring in the body as one 
gets matured.
There is no medical consequences per se due to 
mastubation but i assure you there are psychological
 consequences such as poor sexual life, unconscious
 touching of the genitals even in public areas, isolation
 etc.
To stop masturbation, first the individual needs a strong
will power. He needs to avoid staying alone, avoid watching 
sexually arousing movies and magazines and always try to 
be busy.
Hope this helps. If you have further query, i will be glad to help.
Best regards.
Dr Achuo