It could be any number of things. Vaginas are funny like that. It may not be something she can help per day. Often it could be a bacterial flora issue or STD, but it may not be a medical issue at all.  

Perhaps her diet, or the result of a medication/supplement (I have heard of birth control like Mirena making people’s odors stronger; my mom made lots of food with fenugreek when I was growing up and my body secretions would smell like maple syrup), or her natural hormone levels.  

I am not sure what to tell you about how to handle as I recognize this is a difficult and delicate matter to approach. But in the future, if you are ever I. A situation where you notice foul smell or odor, I would not engage in sexual activity with the person. Just my suggestion.

There can be many reasons for this. It couldve been what she ate, what she washed herself with or her underwear with, things like that to throw off pH balance, sweat, or infections (bv, yeast) from any of these things. If it were me, I would want someone to be honest so I can fix the issue. It's better to suffer slight embarrassment and fix it than be oblivious and embarrass myself more. A gyno visit can fix that quickly!

Hello- while vaginas all have different smells and tastes (diet, products/medications used, natural PH, etc.) there should never be a 'bad' smell per se, at least when a vagina is happy and healthy... it was most likely BV (bacterial vaginosis), a common annoyance which is really just an imbalance of natural bacteria in the vagina, not an actual infection. Of course, I'm not her gyno, and don't know for sure. Hope that helps!