I allowed someone without a TIN to make trades on a brokerage account I do not use. Who is responsible for the tax filling? How to resolve this?

What a horrible idea ... YOU are responsible for reporting all transactions that happen in your SS#.  

I agree, it was a bad idea. And indeed that is what I thought was required for me to do. That the transactions would be reported on my next tax return, but he's making the claim that his CPA says that any gains or losses on my account are his because the funds originated from his bank account. Looking for accurate information/guidance on how to deal with this. 

Thank you for your input

Consult a tax attorney for legal advice.

That is a legal question and not a tax question and we cannot give legal advice.

as an aside, what would the brokerage firm had done had the person lost$1,000,000.  do you think they would come after you?