You represent a dentist who is accused of performing an unnecessary

You represent a dentist who is accused of performing an unnecessary root canal which went bad, resulting in the patient losing the tooth and having to get an implant. 

During settlement discussions, the dentist tells you he wants you to use certain information he claims he gleaned from the patient while he was under the anesthetic during the root canal procedure concerning the patient’s marital infidelity.  The dentist believes if you make it known to the other attorney, the plaintiff will end his case because he is a public figure with much to lose if the infidelity were to become known.  You have serious concerns the dentist is lying to you about what this claim. 

Can you (a) discuss this at all with opposing counsel?  (b) try to find a way to use this without directly talking about the plaintiff’s infidelity?  How do Model Rules 1.2, 1.4 and 4.1 interact here, if at all?