The decision to reject the takeoff is the responsibility of
the captain, and must be made before V1 speed. If the captain is the PM, he should
initiate the RTO and announce the abnormality simultaneously.
This is what Mr Boeing says in his big book of flying jets aka FCTM and that’s good enough for me. More importantly, that’s good enough for my company and that’s what we do. We do train FOs to carry out RTO in the event of the old LHS person keeling over.