Is it permissible to sit with a student of knowledge who smokes and believes it to be permissible due to ijtihad, or is this an issue that does not admit of differing opinions?

The ruling on smoking is a matter of disagreement, and most scholars have deemed it forbidden. If he is a student of knowledge and has become convinced after research that smoking is not forbidden, this does not prevent benefiting from his knowledge. We ask Allah for His pardon and forgiveness for him, as the mujtahid (independent jurist) who errs is forgiven for his mistake, whether in theoretical or practical matters. If you are able to discuss the matter with him to clarify what is correct, that would be a form of sincere advice.