The QRH for the 737 allows for an abort above 80kts for engine failure, fire, predictive windshear warning or if the aircraft is unsafe/unable to fly. I'm guessing the wording will be pretty similar for most Boeing types.
So if the F/O realises his Capt is incapacitated above 80kts but prior to V1 he must make a judgement against the above criteria. The only one that might fit is that the aircraft is unsafe to fly which is perhaps stretching it a bit unless you take into consideration that if the Capt is PF that may be because the weather is outside the F/Os limits (cloudbase,LVPs,wind, contaminated runway etc) in which case you could make a very good case for stopping.
In my company we are very rarely field length limited and often fly off runways with a huge surplus of length so in reality most of the time a high speed abort would involve a relatively small risk. Certainly if I was ever unfortunate enough to fall over on the flight deck on such a runway I would be gratefull if my oppo stopped and got me help asap I wouldn't be berating him for doing it from 110kts as I breathed my last.
The key thing is that the scenario is thought about and trained for and that we appreciate that circumstances will change from aircraft type to the runway in use and the prevailing weather.