I agree with John concerning signatures are not required. But in this situation, since you must provide 2 copies of the rescission form to each consumer having the right to rescind, I would want some sort of documentation the husband received his copies.
Another issue to consider. If he drops the rescission form stating he wants to rescind in the prison mail on or before midnight of the 3rd business day and due to the prison mail processing it's not sent until day 5, where does not leave you? Did he rescind in time or didn't he? As you said hopefully she has a valid POA.
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The opinions expressed are mine and they are not to be taken as legal advice.