Can a poa be required to provide proof if what they are doing with the money of the person they are the poa over

The principal can require her agent or attorney-in-fact (those are the terms for the person given power by the POA) to account for his actions; can revoke the POA; and sue for any amounts or property improperly taken. But only the principal can do this: another person cannot look into what the principal chooses to allow to be done with her money by her agent, any more than they could challenge her on giving gifts to an undeserving relative. It's her money--her choice. You can express your concerns to your grandmother: then she decides what (if anything) to do about them.
The above is the rule for a mentally competent principal, since mentally competent persons can make their own decisions, even bad or foolish ones. If your grandmother is NOT mentally competent, that is a different story: a person, even someone with a POA, cannot take advantage of an incompetent person. But it's not enough that you may feel that she is incompetent: you would need a court to declare her legally incompetent and appoint a legal guardian for her, which guardian could then look into what was done with her money. To declare her incompetent, a legal action, supported by medical evidence (such as the testimony of doctors who treated or examined her) would be necessary. If you want to explore this option, consult with an elder law attorney.