I think you have a good argument for the disabled description in this specific case, since the customer has been disabled for life.
You might have a harder time if you typically just use "unemployed" and do not have a policy of inquiring about their past occupation. In situations where the customer is not present, it could be acceptable, but on face to face transactions, this question should be asked of the customer.
You might want to check out the instructions on the latest Form 104:
Item 13. Occupation, profession, or business. If known, identify the occupation, profession or business that best describes the individual or entity in Part I (e.g., attorney, car dealer, carpenter, doctor, farmer, plumber, truck driver, etc.). Do not use nondescript terms such as businessman, merchant, store owner (unless store’s name is provided), or self employed. If unemployed, or retired are used enter the regular or former occupation if known.
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