I have a joint account with my mum (who is retired). Just today, I found out that has many transactions buying TOTO and 4D, and that she spends more than $500 every month on it. What should I do?

observe for a few months, say 3 to 6 months.

Show her that the money not spent on 4d and toto can be spent on a holiday instead.

then take her for the holiday :)

Tell her to buy toto and 4d from you. Assure her that if the numbers she buys comes out, you will pay her the same amount Singapore Pools will pay. If the numbers do not come out, you can save the money for her and use the money to pay her if she ever win. The chances of her winning.....is low anyways