1)Tell your friend the worse case which is if without the help of doctors.Generally one careless about something is not because one don't know about the 'bad' but 'what the detail of 'bad' is'.
2)Tell your friend 'i don't agree with you' and 'i prefer to do something rather than watch movie...' and make pressure on him with your behavious and a few words.Generally you shouldn't tell him/her parents directly although it's still an option(which is the last option you may choose),Because(and you should remember that) you are a Friend rather than his/her Mom.Keeping secret(unless he/she plans to hurt some one) to win the trust so it can be called 'friends'.