Talk to N and mother first. Open with your personal concern, and experience - to explain why you care, your good intention, and to give significance/urgency to the issue at hand
Offer your help in supporting them in their efforts. Discuss with them how they see it, the state they're in, what they can do and influence.
I'd consider two approaches - not one or the other, but chase/asses both.
- Limit access to funds, secure funds for needs
- Help them (G) with their addiction and spending - this is very dependent on their personality, view, openness, and personality + your approach to them. Working together with N and mother is essential
If they don't want your help, or don't see any approaches as feasible, accept it as it is and that you can only do so much and not help everyone even if it's in your capability and interest.