In most cases, you see earlier than the gambler that things are going wrong. Or you suffer in your own way because the other one gambles. What can you do?
Leave the consequences of the gambling to the gambler; let the gambler solve the problems caused by the gambling! You are not helping by lending money, paying debts or lying. The more problems you solve, the easier it is for the gambler to continue gambling.
Talk about the consequences of the gambling for you: it is important that gamblers know that their gambling behaviour has consequences for family and friends as well. Make clear what you find unpleasant and why: I don't like that you come home late because then I can't count on you, I don't like that the bills are not paid because then I worry about the debts, I find it horrible that you are not honest to me because then I can't trust you, I find it difficult to deal with your restlessness because then we are only fighting.
This way you will make clear that the gambling has (a negative) influence on you and you put the responsibility on the gambler.
Also show understanding. No matter how angry or disappointed you are, to reproach somebody is usually counterproductive. For people with gambling problems a small provocation, like walking past a gambling hall or having money, can be enough reason to go gambling. Once they start, it is hard to stop. Let the person know that you are prepared to help and to talk about the difficulties, but be clear that the consequences of the gambling are for him or her. Together you could find out what should be done to get out of trouble.
Make rules and determine limits. You can probably understand that it is hard to stop gambling, but that doesn't mean you can accept everything. What you can do is look at what is necessary to change. Look at how you want things to be, not how you don't want them to be. For example: "I want you to be honest", instead of "I don't want you to lie". "I want you to keep your promises". "I want you to find a solution to pay off your debts".
Take safety precautions. Give exact money. Hold bank cards in trust. Or arrange a deregistration in the casino or gambling hall altogether.
Seek professional help to learn how to deal with gambling addiction.