Hi Nannie Payne,
According to the Social Security Administration, both you and your ex could be charged with fraud because he failed to tell the SSA that he had returned to work and continued to collect benefits.
http://www.ssa.gov/oig/hotline/facts...htm#concealing
Allowing use of your SSN makes you complicit in the fraud.
Under
SEC. 208. [42 U.S.C. 408] (a), this is a felony, and is punishable by a maximum fine of $250,000, five years in prison, or both.
It would be in your best interests to contact a local attorney. Good luck!