Now knowing about the ammonia/hot tub, this post probably won't apply here. But it might help anyone else getting into a similar situation..
Right after my second son was born, I got some kind of infection. The smell was AWFUL! I never got used to it, neither did hubby. Maybe because it didn't smell like hot restaurant bread? LOL.. anyways... it was so strong at times that I could smell it through my clothes. I bathed 3-4 times a day, went back to the doc 3 times... nothing. Finally, I switched docs and this one listened to me, smelled me I'm sure, and treated me. But it was months later. Months now, no oral.. waaaa it was horrible!
Anyway, our experience is that a very BAD, odd smell is likely infectious in nature, and if you are one to go bareback, might want to pack condoms in the "emergency bag" lol. And not just for hubby... what she has, her hubby has!
If it isn't that kind of smell, or urine, then you can probably bet it has to do with diet or her cycle. Other causes can be medications and illnesses.
Just as diet changes the taste of sperm, it changes a woman's smell. I can eat seafood weeks on end, but eat shrimp? oooo bad smell lol... Turnip greens.. yuck.... It's not too many foods, mind you, but there are a few.
Then there is her cycle.. the closer she is to her period, the stronger the smell. Right after her period, all roses lol... well close!
Oh and as for telling them.. I understand what the above links are saying but I don't think it applies here. I don't smell my pus all day long lol, to be desensitized to it. If it is a little "odorous" for the most part it hits me when I sit down to pea. So I have a good many hours of the day I don't smell it, then a nice kick in the nose for a few minutes every several hours. Can't miss it. So if it is a long-standing problem, she knows. And, she should have told you ahead of time - so on that note, I say it isn't worth the stress of telling her. If it's not a long-standing problem, it likely will disappear in a few days.. and why embarass her?
Hope that helped some