Can I sue my ex main tenant for fraud

I was living in WG for two years. I recently found out that the original contract (between him and the landlord) he shared with me was fake/forged. He inflated the actual rental amount to justify high rent of the apartment.

I knew that my main tenant was charging me more than 50% of total warm rent, which I made my peace with but this fact that the original contract is fake and basically he lived off of me in that apartment seems like a fraud. Can I sue him for this fraud?

Any tips would be helpful. Thank you.