N52
In your senario I would pass traffic information and leave the pilot to decide a course of action - he could ask for an avoiding action turn. The climb above the RVC may not be an option due to other traffic. The traffic information should be passed as controllers we have a 'duty of care' to ensure pilot/aircraft safety. Should it all go very badly wrong the question you could be facing is did you do all that could be expected of the average controller in the conditions - difficult one to call.
Regards
P