I thought it was obvious, but: when there is no further action he can take on his own that would help clarify the guilt of the accused.

So it's just the effort he makes to be certain, not how certain he is, that is important to you? Interesting. Should we all start throwing out names just in case if we just make reasonable effort? I have plenty of improbable accusations to make.

He doesn't, and I wouldn't require that (a proof) of him.

You must be friends then. That doesn't help me to judge the veracity of his claims.