Friend needs help want to know what i should do

Ok so my friend who looks fine weight wise thinks she is going to get fat by eating. but she does weigh more than average but as much as me and the rest of my friends say its just muscle she doesnt listen and doesnt eat as much as she should. one of my friends saw her take diet pills. we are all quite worried about her health. the same friend who saw her take diet pills told there parents after consulting with me and some other people. was this the right thing to do or should we have done something else.
Also the parent who we told is going to tell my friend who is not eating parents

Of course its the correct thing to do. you would be a bad friend if you didn't help your friend out. they may think you are a dick in the beginning but they will understand later on