With certain exceptions, by statute (see CPLR §1004) and common law in New York (see Spencer v. Standard Chem & Metals Corp., 237 N.Y. 479, 480 (1924)), the real party in interest must be joined to an action regardless of whether the plaintiff that commenced the action had standing.

Having standing is not equivalent to being the real party in interest; having standing is equivalent to being an agent.  In addition to CPLR 1004 and the common law, the agency principles of ratification and repudiation mandate proof from the real party in interest itself.