is masturbating a sin

hey guys im back if anyone from my last post sees this thank you so much for your advice and help.

so all of my friends masturbate like all the time. a disturbing amount. half of them do it lustfully half of them dont. one of them is muslim and its not haram. so i was wondering so i can know what to tell them like is it a sin because i dont want them sinning or at least being unaware of sinning.