Have the client call healthcare.gov and see whats on file for theirs or one of the SSNs on their tax return.    IRS has been informed that a 1095A has been issued to someone on that return.

Seems more common than one would think that people inadvertently sign up while looking for a quote, and it doesnt cancel for non payment.

If your client is sure, and they check on healthcare.gov that they don't have it just fill out a dummy 1095A with zero in box 1 and 2, and a $1 in column A,B,C. Then resend. I had two of them this year, and this worked fine.