Your lawyer is right that joint owners of bank accounts both have full and equal say in how funds are accessed and spent.
But there is another dictate that makes
it illegal for one person to come in and defraud another person"”that is, for you dad to take your mom's money for his own use and enjoyment against her will, or when she lacked the legal capacity to agree to his expenditures. That's elder abuse, which extends to abuse of finances as well as physical transgressions.
There are a couple practical concerns that you might want to consider before taking legal action in this situation. First, you will need some fairly strong proof that your dad took the money wrongfully and spent it on something other than her care or best interests. If it comes down to a lawsuit, he would likely be required to submit an accounting detailing how and where the money was spent. But even before that could happen, you must be able to produce some fairly bulletproof evidence that he overtook her funds when she was not able to direct them meaningfully for herself.
Another practical concern is that taking legal action against your dad might well mean the end of your relationship"”or is at least likely to add difficulties to a relationship that may already be strained. If you feel your mother was truly bilked out of her money, this shouldn't prevent you from taking action. But it's something you might need to weigh and balance.
That written, if you believe your dad has committed financial abuse by misusing your mom's money, there are a few sources to which you can turn for help. Contact your locale's Adult Protective Services, call the Office of the Attorney General, Bureau of Medi-Cal Fraud and Elder Abuse at 800-722-0432, or consult an experienced attorney for help.