Assume you are working on a 12/31 year-end audit

Assume you are working on a 12/31 year-end audit. It is now March 31st and the 12/31 accounts receivable aging shows a large receivable that was outstanding on 12/31 for 120 days. Further, the client's receivables are typically collected in less than 45 days. You anticipate that the client's allowance for doubtful account should be increased and inform the client about your disposition. The client disagrees. Is there an alternative substantive procedure that you could perform that would provide convincing evidence that this balance is collectible? If so, explain.