She's wearing a see-through dress, and you know she doesn't realise. What should you do?

xyzzy
Case example: a girl in dance class last night (being a Christmas party and all) is wearing a red/white christmassy dress - but it was definitely a dress that's supposed to be worn with a slip, plus she chose underwear colour poorly. Her outline was clearly visible (let alone the underwear). Would you say anything? Should you? If you chose to mention it up, what's a tactful/polite way? If you are a girl, would you want to know (esp if it's a guy telling you)?
She's wearing a see-through dress, and you know she doesn't realise. What should you do?
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