You were never PIC so you were correct not to log anything. You took over the actual flying of the plane with the permission, and under the supervision, of the PIC. They could take back the controls at anytime they liked and if you put the aircraft in a situation outside of their competence/experience level, but within yours, all you could (legally) do would be to offer advice which the PIC could accept or reject as they saw fit.
Totally agreed. So how is that different from a club asking me to go and ride a circuit or two with a new pilot, to make sure they're okay to rent to? I tell new pilot that they are PIC, and I will discuss with them in advance any action I might take to take control of the aircraft. 'Seems the same either way to me....