Jan,
What would it matter if your friend believes this?
Let them believe what they want.
They have the right to be wrong :-)
You could simply agree with them to let the subject be. It seems mere facts won't change their mind anyway. They have a preset belief, and every fact anyone offers that contradicts that worldview will be rejected as tainted information. It will also likely push you two further apart if you argue about this.
Just be their friend. Don't discuss the subject anymore. If it becomes an obsession for them that actually threatens their health you could address that (without arguing whether they are right or not).
After all, if Bill has everything in their power your friend wouldn't be able to expose or fight him anyway.
Best thing they can do is choose a different charity to donate to.
If they have to obsess with every crook around (regardless of whether Bill is one) they'd have no life...