Perhaps try to shower or bathe with her, it's a very romantic way to innocuously clean her up down-there.
After the shower, revel in her cleanliness, lavish compliments upon her about how she smells so good, especially 'down there'. (Only do this if the shower worked well
) Hopefully this will encourage her to keep her personal hygiene a little more stringent. Lather, rinse, repeat. As in, if she doesn't get it on the first try, and later on still smells bad, don't lavish attention on her 'unit' until after she's cleaned. She'll figure it out eventually.
Otherwise, like, if washing doesn't clean the smell out, that would be time to have a serious and frank talk with her about it, because it could be something affecting her reproductive health. If she takes offense to you worrying about her health, seriously consider whether you want to date someone who doesn't trust your intentions.
How Do I Tell? 
There's really no light way to bring something of that nature up, unfortunately.
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