Mother with a gambling addiction. How to stop it?

You can't. Only your mom can get the help she needs. But you could check out a gambler's anonymous meeting and offer to go with her. And, you could offer to go to an addiction psychiatrist with her for her first visit.
Treat the cause. I wish it were easier and simpler;however, as with an general symptom, many causes can lurk. Not to scare, but a thorough psychiatric evaluation would be the best mother's day gift. Offer to go with her and to find one. Most likely, this is psychiatric in nature : a form of anxiety, compulsive, even phobia (of death); but slim. Physical cause/contributing factor. We are complex beings.
Gambling. She has to want help, first thing;s first. I have heard mixed reviews about Gambler's Anonymous. I would say have mom seek psychotherapy to find out if the gambling serves an emotional need she may not be able to speak of yet.
Mutual support helps. You can not control your mother's gambling. You might consider attending a meeting of gam-anon for the families and friends of compulsive gamblers.