If her personal hygiene seemed good she may have a dose of BV or bacterial vaginosis. Treatable with over the counter at chemist. It is not a reflection of her personal hygiene… it’s an imbalance of good / or bad bacteria.
Possibility of getting UTI’s, Thrush or BV I think is higher when you’re having a lot of sex (especially when you first start sex work)… even though they arent actually STDs. Eg my friend gets Thrush from Lube, Antibiotics can cause dramas as well.
Not sure how you would put it… She would realise herself at some point, but you may have realised before she has if you’ve ventured south to dine at the Y.
Perhaps something like…
‘Hey xx. I thought you were amazing and your hygiene seemed good… so not sure how to put this, please don’t take offense, but it was a bit wiffy down stairs…my ex girlfriend used to get BV sometimes, maybe check it out hun… would like to return when you’re 100%’
‘I hope you don’t take my message the wrong way, just trying to be helpful as you seemed lovely’
etc/ so on
good luck…. And we’ll done for not trashing the poor girl out x