Hello,
My Husband won custody of Daughter at age 4, Mother proven unfit by State. Daughter is now 28. Father would like to sue for back support now, since Mother has inherited 3 Million Dollars.
Is this possible, we all reside in NY State

Was a support order ever filed? Its too late if one was never filed.

Did he go to court when he got custody and get an order for support that was not paid? If there is an existing order that went into arrears then yes, the back support is still owed. However, if there was never a support order then there is nothing owed so the answer would be no.