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Onion smell from vagina can be due to fungal infection which is quite common,intake of food containing more of onion,garlic,vinegar,mayo etc.
Ask her to change the diet if she has been taking these products.Wash the genitals with tea tree oil in water twice a day.Avoid tampoon use,douching,bubble bath etc. and observe for changes.
If the symptoms persist consider visiting a Gynaecologist for evaluation.
Take Care!