There has to be absolute clarity on who is acting as PIC before the flight. There can be no shared responsibility here, the person nominated as PIC is responsible for the flight, especially if there is an incident/accident.
The OP can provide some “advice” to the other pilot but if he is expecting that this advice must be followed then they are the PIC and the other person is a passenger.
There was a significant accident at my home drome years ago in a similar situation. The landing started to go pear shaped and the pilot flying expected the right seat pilot to take over and fix it. The end result was no one was flying for the critical few seconds and the airplane was rolled up into a ball by the side of the runway. There was no preflight discussion on who was doing what and as result an airplane was wrecked.