it's a bit your words against his/her words.
except that your reviews and the reviews your guest received might make the difference.
if you only have good reviews, high cleaning ratings and are recommended by your previous guests, airbnb will see it and should take it into consideration. And if the guest is not recommended by other guests, airbnb will should also take into consideration.
If the room was (really) dirty, the guest should have mentionned it to you right away to fix it, or complained to airbnb to get a refound or relocation.
I dont think, taking, before/after pictures for each guests is really usefull, can't proove the before picture is really just before.
I'd be curious to know what airbnb tells you and wich action they will take.