OK, you need to get full Power of Attorney or guardianship before she gets so far gone that you can't and the courts intervene.
With full POA or guardianship you can stop the donations and preserve her funds as she will for sure need them in future.
Hit up your local lawyer.
My $.02.
This is the right answer. Let her know that you're concerned about her running out of money and offer to take over her finances. Unfortunately this will create more work for you, but you'll be able to preserve her wealth. You can tell her to make a list of organizations that you'll donate to after she's gone and no longer needs the money for herself.
Don't bother to argue about religion or discredit the preacher. Anything you say that runs counter to her existing beliefs will be dismissed as Satan's attempts to deceive her.
Or he has brothers / sisters, which would make the issue more complicated.
He can either get control of the matter or let things go to Hell.
His choice.
Thank you for your kind reply!
My $.02.