What can I do about my son taking advantage of my mother?
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That is a sad story. First, obviously, the son's actions have to stop - and I assume you have cancelled the ATM card and/or otherwise cut off his ability to withdraw this money. If you have not, that must be done immediately. If you need to hire a lawyer to accomplish this, you should do so. The most important thing is for the theft to end.
Consequences to the son, and recovery of any of the money is a whole other ball of wax - and I guarantee a monumental, long-term headache for YOU, so give yourself a cooling-off period and think long and hard before taking any further action. You are really mad right now, which is totally understandable.
You could report the whole thing to the police, and get your son arrested and possibly tried on some sort of theft charge. This is not a slam dunk if he insists your mother allowed him to use the card, since the dementia has set in and your mother may not be competent to testify at trial. If convicted, he could do time in jail or probation, possibly be ordered to pay back the money, etc.
Getting the money back (by way of garnishing the wages of the son - I assume he has a job, but no other real assets) would require a civil suit for the misappropriation of the money, which would require you hiring a local attorney who would have to be paid from any proceeds collected (and possibly some up front). You cannot get blood from a turnip - meaning if the son has no assets and no job, he won't be able to cough up any money to repay your mother (court order or not).
As for the others who have taken advantage (the gardener, etc) you should forget about this, as long as the behavior has ended. It is highly doubtful you could get anywhere in court with such claims.
Good luck, and I hope you can reach a resolution at some level where you feel at peace.