How do I tell her to lose weight?

So I work at Abercrombie & Fitch and along with working in store, I'm expected to let all the new workers know about the new routines etc. Yesterday my boss told me that if one of the new girls keeps gaining weight then she'll be fired. How am I supposed to tell her that she's getting too big? I don't want to hurt her feelings
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I don't want this question to meme me look like a shallow bitch, I don't want to offend her but I don't want her to lose her job?
How do I tell her to lose weight?
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