The lawyer's role is not clear from the posts. If this was a private sale and the seller employed a lawyer in order to facilitate the sale process and closing, the lawyer is not an agent within the context of the statute. The agency relationship between the lawyer and client is not the same type of agency that the statute describes when it speaks of "enter[ing] into a contract with an agent for the purpose of selling or leasing a unit of target housing" -- that's a person acting in the capacity of a real estate agent.
If the seller did not provide a disclosure as required by 42 U.S.C. 4852d, then the buyer has potential recourse against the seller under that statute, and possibly also under state seller disclosure laws. But as it stands, no information has been provided that would allow us to say that the lawyer operated as an agent as contemplated by 42 U.S.C. 4852d so as to share potential liability.