Wow. That sucks.

I am not a lawyer so I cannot give you a good
answer.  

I do have a friend who was in a similar
situation and was able to get out of it because
he had smoked pot with his ex-wife before (on multiple
occasions) and he pointed the finger back at her
until they both signed an agreement not to 
bring it up again and not to smoke when the 
kid is around.

.....

But my real advice to you is to shop
around for a really good lawyer.  Keep
looking until you find a lawyer who
is very confident that he can help you.