Can I become power of attorney for my aunt who has an only child who’s abusing her money?

If you have solid proof of the son's stealing money from his mother - that is without her consent - you can call Adult Protective Services. It can't just be your hunches that he is taking money without her knowledge. Could it be that she allows him to handle her money? And his way is not the best way but she gives him permission nonetheless.

Medical POA is just that - medical. A Medical POA has no control over financial matters. You have to have Financial POA or a dual POA (covering both medical and financial) in order to control both matters.

As Jeannegibbs mentioned above, if your aunt is mentally competent and able to make her own decisions, she has to be the one to decide who gets POA. If she chooses to allow her child to continue to handle things, your only option would be to gather proof of what you say is happening and talk to an attorney to see what your options are.

Is that what your aunt wants? She has to be the one who decides who her financial and medical POAs will be.