What do you do when you know someone is stealing from someone that is handicapped and needs help?

If you think there is abuse going on you can report it to Adult Protective Services.
There might be more to the situation than you are aware of but I am of the mind better safe than sorry. All it takes is a phone call and you have a clear conscious. Let the professionals figure out the situation.
If you are SURE there is theft then tell them what you're aware of.

Does the woman take care of the woman who is disabled? If the disabled lady was in a nursing home, she’d be paying upwards of $5000. If she can’t use the kitchen, does the woman cook for her? The kitchen may not be handicap accessible. Stoves have the controls on the top and she probably can’t reach them. Same for the sink. Can she use the microwave? Does the woman buy her food for her? Do her laundry? Take her places? Has she had to make any modifications to her home for the disabled lady? The disabled lady has her own private caregiver. Unless the woman is actually taking money out of the disabled lady’s purse or bank account, it’s not stealing. If you speak with the disabled lady, ask her if she feels she’s being stolen from. If she says yes, offer to help her find another place to live.