Just tell your friend straight up that she's being fucking stupid. If you really want to lose weight why not be smart about it and do it the right way? It's not even that hard. It's about calories in vs calories out. You want to lose weight? Expend more calories than you consume. I think I speak for most guys when I say we prefer the looks of a fit girl rather than a "skinny" one. Count your calories, count your macros and lift or swim or run or whatever. I dare say a big part of it is the attention too. Let her know she's being an idiot, but don't spend too much time on it, deprive her of the attention.
If all else fails, talk to your school nurse or councellor or whoever's job it is to look after health and mental health.
If all else fails, talk to your school nurse or councellor or whoever's job it is to look after health and mental health.