Obviously the family has sufficient money. Why should he labor to earn money that is not needed? Just to build up a bank account. What is the point in that? I suspect that dying with a lot of money in the bank will prevent you from going straight to heaven. I don't see why it is a sin at all. There is no commandment: Thou shalt support a capitalist with your labor. And is it not a rash judgment to say he is idle? Do we know how he spends his time? It seems that he does assist at Sunday Mass, we can probably assume that he also prays the rosary.
It seems to me that you are taking too much interest in what this man is doing. How is it your business? Trying to find someone guilty of sin seems to me to be something you will have to answer for at your particular judgment.
Really appreciate coming at this more from the direction of tolerance, understanding and empathy rather than the opposite - this is one of the reasons for posting - thank you!
I apologize also for appearing to change the proposed case from the objective to a more subjective one by adding unnecessary details; the original intent was to solicit the response for the more general objective case where there's no circuмspection involved. 
Let's return to that more clear cut case and ask again, when, if ever, does chronic unemployment - without any excusing factors - objectively speaking, become a sin, possibly even mortal?
Or is this a strictly temporal phenomenon without eternal/moral consequences beyond St Paul's admonition, "...if any man will not work, neither let him eat." 2 Th 3:10, " ? *
Spirago has some other fine Bible references on this, but I was wondering if there is also any notable teaching from Church Fathers, ref. the capital sins, etc about moral culpability for not working (either physical or intellectual labor).
I am pretty sure we don't need an 11th Commandment exclusive to work to know that not working is contrary to God's Will, and any true friend of such a person would be conscience bound to offer fraternal correction in the pattern recommended by St Paul ("And if he will not hear thee, take with thee one or two more..." Matt 18, 16)...
... but when does it become a sin, and more importantly when does it become mortally sinful?
*Disregarding also, for the moment, the previous reference to the double commandment inherent in the 3rd Commandment, i.e., "Six days shalt thou labor...but on the seventh day is the sabbath..." etc. Ex. XX 8-9.
"Ora et labora"
  - St Benedict