How do you stop a woman online from asking my elderly dad for money?

Is she asking AND he's giving? Or does he just ignore her requests? If this is a Nigerian scam, report it to the Feds. In fact, do that anyway.

Report her to her ISP. It can terminate her account. Of course, she can just switch ISPs.

How did he meet her?

How good are you at "adjusting" settings on your father's computer?

Can you give more information about the situation? Is this woman emailing your dad? If so, can you "BLOCK" her email address. How well does your Dad know this woman or understand what this woman is doing?