According to 1.1 General definitions:
“Flightcrew member means a pilot, flight engineer, or flight navigator assigned to duty in an aircraft during flight time.“
I can’t find the definition for “required flightcrew member”. So assuming that it’s dependent on the type aircraft, we all know the C172 only requires a single pilot. But it doesn’t limit you to one. It’s not explicitly stated, but the intent is clearly “at least one”. Nothing prevents you from having another equivalent crew person aboard.
But notice that “flightcrew member” says nothing about who is PIC. The requirement is that they all be aboard and certified, rated, current.
Also notice “assigned”. When you take out your 172, or rent one, you are self assigning yourself as the pilot (the required crewmember). But if you have a qualified friend along, there is nothing preventing that friend from also being a “crewmember”. A C172 requires at least one qualified pilot but does not limit it to one, and, does not require that crewmember to be in the left seat.
Therefore, because the definition says nothing about who is acting PIC, both can be considered a “required crewmember” for the entire flight! But, to use an electrical analogy, they are in parallel, not series, so that if one fails, the other is sufficient.
My first analysis saying you had to say “My airplane” and take over PIC was wrong. Once the first pilot becomes incapacitated, the other pilot is “assigned” the duty by default.
Therefore no report is necessary. You had a spare required crewmember aboard all along.