look, its like this.
the F/O most certainly could have landed the plane without aid. current CRM thinking is to use all available resources to ensure the safe outcome of a flight, especially in an emergency. (crew illness is an emergency)
a request for another pilot is prudent.
as to taxiing. the first officer can easily control the plane on the runway with the rudder pedals which actuates nosewheel steering.
but the amount of nosewheel steering in this manner is somewhat limited and might prove insufficient for tight turns into parking/gate area (apron or ramp, not TARMAC).
it would be normal for the F/O to continue to fly the plane from the right seat to landing. after the plane was safely stopped on the runway, a decision based on available resources is the prudent way to act.
portable stairs for the paramedics to meet the plane on the runway and a quick shutdown of the number one engine (port) would get medical help to the ill person the quickest.
after the medical help had taken care of the ill person, a decision to move the plane would be made.
either:
wait for a tug (most prudent, except for time)
start the engine again and have the person in the left seat taxi the aircraft (the copilot could switch seats) or allow the acting pilot in the left seat to handle the controls (not what I would do). go to the gate.
There is a possibility that the plane went straight to the gate for medical help in which case taxiing by the first officer in the right seat would be prudent, allowing for the temp/acting pilot in the left seat to use the tiller in tight places.
Remember, many gates have a straight in approach for some distance and it could be handled by rudder pedal steering.
details would be helpful.