You don't. You can't solve other people's problems for them. All you can do is provide space and support for them to solve the problems themselves.

Agree with the comments about discussion as well as not changing other people, but as you describe it as a dopamine hit, that signals addictive behavior to me.

I would suggest slowly introducing information regarding how addiction and dopamine works in humans. Allow him to learn more about it, perhaps as a reciprocal gift of books, shows, etc.

Ultimately he'll need to wean and change his own brain chemistry, and that is not easy.

(If I get time I'll dig up a few articles on it.)

Do they have a partner or a close family member you could talk to? When I had a spending problem the only person that could help me was my partner. I put them in charge of my finances,l – gave them my cards, made sure to delete saved payment methods online – so I had to justify purchases to them first. They’d never say no to anything, it was my money after all, but having to justify my spending and taking a moment to consider really helped.

Have you told him the things you wrote in this post? That might be a good start. You could also suggest a substitute. If it's some sort of addiction he needs to work on, maybe he could try to donate money to a good cause or even buy a gift for someone who needs it (local charity or something like Savethechildren.org). Might provide that same feeling and not cause you worry.