A breach of a written contract has a four year statute of limitations. Breach of an oral or implied contract has a two-year period. The failure to bill promptly should not be a defense in court, provided that the contractor can prove to the court that the work was done and done properly, which it appears the customer will admit it was. If we are talking about several years since the work was done, a judge may find that it was done as a gift, if there is no express contract with the customer. If the amount is under $7500, consider small claims court.