I respectfully disagree with attorney Potter--presuming that this debtor is a life tenant (i.e. the person who has a right to possess the property for the duration of their life), that life tenant does have an ownership interest and a lien can be filed against that interest. The "right to live there" is more akin to a lease, which isn't applicable here.
The interesting legal question is whether a lien established by the debts of a life tenant and not related to the property (like a tax lien) is extinguished by the death of that life tenant (on the theory that you become a creditor of their estate instead, and that there's no remaining interest in the property because it went to the remaindermen). Advice on this point would be recommended.
Simply put, a lien on the life estate would mean that in the event the life estate is sold, or if the life tenant and remaindermen join to convey a fee simple interest in the sale, you'd be paid out of the proceeds. If the life tenant property is rented, New York provides a method by which you can intercept the payments due to the life tenant.
I am licensed in New York only. My answers are generally based on New York law or common law, except where otherwise indicated, and other states may make exceptions or have peculiarities resulting in different answers from an attorney licensed in the other state.
His life estate merely gives him the right to live there. He has no ownership interest. Consult a Real Estate attorney in your area to go into more depth on this issue. See Avvo.Com under Find-A-Lawyer