I have a friend of a friend who had a diagnosis of having a shopping addiction. She spent money to make herself feel better (her father and cat both died within days of each other and she had other emotional issues). She had credit cards her husband did not know about and her spending was so out hand, that the couple (who have 2 kids) were at risk of losing their home. She also did not work so was not contributing to the household income.
She was admitted to a mental health clinic for treatment which included setting up a budget and giving her a set amount of money for her personal spending (above and beyond the cost of her daily living expenses).
She is held to the allocated amount of money - if she spends it at the beginning of the month - too bad, her loss. It’s not been easy for this couple, but at least they’ve taken a step in the right direction by setting boundaries and holding her accountable.
Your friend sounds like an enabler - he needs to sit his mother down, tell her what he can afford to give and she needs to budget accordingly.