Try and steer clear of ultimatums. Ultimatums are rarely effective and they can increase the sense of guilt and shame a person feels about their behaviour.
Making ultimatums may encourage greater secrecy and rarely prepares someone for change.
Generally ultimatums are made out of utter frustration, desperation and anger. When everything else seems to fail in making our loved one realise the damage they are causing, we believe that if we can force an outcome serious enough, that our loved one will suddenly, finally be compelled to change. You feel you need to take definite and immediate action.
An ultimatum may work temporarily, but then the addiction overcomes their desire to obey your ultimatum. It always returns, no matter how forcefully you try to push it away, no matter what tactics you use. Your threat has made no difference.
You should always think carefully before doing anything that enables the gambler to continue to gamble. Lending money or lying about their gambling helps problem gamblers avoid the consequences of their actions. They should take responsibility for their own behaviour.
Speaking to a qualified counsellor or even calling one of the problem gambling helplines might be able to assist you plan your approach in advance. They will have come across this situation many times before and can help you decide on the best path for you.