My suggestions are foundated from my own personality type and belief systems
I am a compassionate woman...
So, with that said...
I would avoid putting a damper on the other persons' libido
I would not attempt resolve this situation, in a course manner or make a funny peeeeeeeeeeeeeew faces
That would ruin the whole experience in entirety
And in my opinion it would be mean to say..
"Hey your a$$ smells...You need to wash it!!"

Possibly suggest offering a special sudsy erotic shampoo??
Or maybe a joint venture to the shower....
OR just grin and bear it!
This business involves intimacy...
Like any business there are ups and possibly smelly downs
Intelligent tact and resourceful approaches do prevail!
Good Luck!
Stacy