Once durable POA is revoked, does the person have to have a new POA?

https://www.legalzoom.com/knowledge/power-of-attorney/topic/revoking-power-of-attorney the shortbread version. You probably have it all fixed by now tho :)

She can hire a firm that has licensed fiduciaries, typically they do not charge unless they are performing duties as POA.

You are a caregiver, yes/no? You know that she is being robbed and it is now your responsibility to protect her, you need to get the POA revoked and send the revocation to everyone who has been given a copy of the POA. Simple letter with her name, POA information and state that it has been revoked and any business transactions after date of receipt will be fraudulent and they can and will be held legally responsible for any/all damages. I would send something simple, have it notarized and mail certified return receipt. Then get her to an elder law attorney and have them do a letter and send it out. If you can see an attorney right away, you don't need to send the simple revocation notification, however, if it is going to be enough of a wait that the current POA can clean her out, you should do the letter.

You should file a report of financial exploitation of a vulnerable senior and any other illegal actions they have done.

If you are licensed to be a caregiver you are a mandatory reporter and could lose your certification and license if you don't, if you aren't licensed you should report because it is the right thing to do. If nothing illegal is happening no one will be hurt or go to jail, so you don't have to worry about that.

She can assign a new POA if she is not incapacitated. Does she have an elder law attorney? If not, she needs one, now. Help to get her in to an attorney.

Yes, she should assign another POA. Even a lawyer better than none at all. She needs to revolk and then prove they stole from her and bring them up on charges. POA does not give them the right to use her money foe anyone but her.