I've encountered that kind of resistance before as well. My normal technique for handling it is to have ruled it down to an issue with their component, but instead of hitting them directly with a "the problem is in your component" (preceived) frontal assault, I instead as them to verify the behavior of their component on the grounds of ruling it out as a part of the issue, while making it clear I am still examining other potential issues. This allows them to discover the issue, rather than having it be pointed out to them, and gives them the opportunity to save face. (It also allows for the removal of my foot from my mouth if instead I find there was something else I missed.)

YMMV.

(And I do agree with the appreciation of straight talk and lack of excuses. Your fault, my fault, nobody's fault-let's get the problem solved, and if anyone wants to talk about the dust motes in the eyes of others after having de-planked their own eyes when the current fire is over, they're welcome to.)