I believe it’s quite possible DM had not filed his taxes for no other reason than a lack of life skills. It’s possible someone else had done it for him before, like, say, his ex-wife. Sure, it’s a lot of money lost but it was MBM’s loss not his. He seemed fine with the arrangement in which they received gifts of groceries and firewood etc. from family members. Maybe a MBM without income was easier to hold onto when it seemed she was wanting to leave. It’s also possible he picked up a little income here or there from other employment we’ve just not heard about.
He also seems like someone who was content to live a simple life, and that doesn’t cost a lot when you inhabit your mother’s trailer.
Just some thoughts and possibilities.
IMHO