I was under the impression that larger aircraft carried two pilots so that in event of one pilot becoming incapacitated, the other would be capable of flying the aircraft to its destination, albeit with a reduced level of safety. Modern aircraft are complex, but we are frequently told that the pilots only hand fly aircraft at take off and landing. It isn't as if this flight would have been hand flown across the Atlantic by a single pilot.
Indeed they can, however, I suspect this is more to do with "seeking medical attention" for the affected person, rather than flying "solo" across the pond.