I agree, Redleghunter.
My father was in a position where oncologists refused chemo because they thought the chemo would hurt him more. The insurance company didn't refuse to pay for chemo; the doctors were making that decision.
If the pastor had a heart condition that could be worsened with chemo, an oncologist might decide not to treat him. He's receiving chemo, so his oncologist must have no objection to him receiving it. So, why would the insurance company object?
It really sounds as if, like I think you suggested earlier, his insurance plan through the church just was not a very good one. Hopefully people will help this pastor, run a fundraiser, etc., to help him pay off that big bill. Then he can focus on just getting well.