This incident was discussed on R&N for a while. Consider:
- The V1 speed is artificially reduced to be below VR, as such the V1 speed is NOT the maximum or limiting speed which it is possible to reject the take-off, if the aircraft wheels are still in contact with the runway (i.e. the V1 speed used for take-off is not necessarily limited by either runway length OR brake energy limits) - that speed limit isn't calculated, but can be intuited by an experienced pilot.
- The V1 decision is only trained for engine failure - not for control problems or un-flyable conditions. If the aircraft is considered un-flyable, then a reject at ANY speed is appropriate.
- In the same sense that a Captain may disregard SOPs in an emergency situation when the Captain considers the aircraft is under threat - does not the same apply to a licensed and qualified FO, when the aircraft is considered under threat - and there is no time to consult with the Captain?
- In this case - the aircraft stopped, and taxied back without problems - so 20:20 hindsight says that the FO didn't endanger the aircraft by stopping.