So, this might be an unpopular answer, but frankly, I'd want to fuckin know. Please gents, if you EVER show up to see me and there's a smell, fuckin tell me!!! Be NICE, but tell me. A lady needs to know something like that. She will be embarrassed, and might get defensive (depending on the lady) but at least you know you did what you could to help. All you have to do is on your way out the door, stop, turn around, and tell her "You know, it wasn't a deal breaker for me, and I had a great time, but there was a bit of an odor down there. I'm sorry, but I thought you should know." That's it. Not that hard. And if she gets pissy, well, you're already at the door anyway, just walk out.

Personally, I'd probably send a thank you email (as soon as I got over the embarrassment and fixed the problem) and offer a discount to come back and try again.
Seriously guys, you need to be able to speak up. I sure as hell want to (and do) speak up when one of YOU is a bit smelly.