How do you tell your friend an outfit looks bad?

My friend had a custom dress made for an event coming up in a few days and it really doesn’t look good at all. I don’t know how to tell her without sounding rude or hurting her feelings. The dress is too tight and it isn’t flattering for her body shape. She has a really wide back and an apple shape. The dress is really too tight and I don't know how to put in words that won’t sound rude. A lot of people will be there and she has already paid for the dress. Maybe I just shouldn’t say anything at all. If it were me , I would want to know.
How do you tell your friend an outfit looks bad?
How do you tell your friend an outfit looks bad?
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