The ten commandments and stuff...

Im confused...my aunt was away for a week...upon her return she is hunting for the phone which she finds in her fourteen year pld sons room...Upon finding the phone something promted her to go through the gallery... She found pictures of said son naked with his erect penis and pictures of other naked chicks...anyway point is she is very traumatised and thinks her son is the spawn of the devil...Anyway so the big question among the mothers is "why was he masterbating?" I put it down to being old fashioned...but after having a few discussions with my friends the question became "is masterbation a sin?". Is there anywhere in the bible that it says do not masterbate? Someone HELP!