"Certain to be" is the right answer. It doesn't say anything about if Ronnie knows. It is still just the first person's opinion.
He could be certain because he knows the hiring manager. He could be certain because of information he got from Ronnie. Ronnie's knowledge is not relevant to this question at all.
And if you say "In my opinion, Ronnie is certain to be offered the job" it does not say anything about what Ronnie knows or does not know.