Doesn't matter a damn whether he was allowed, qualified/unqualified, able/unable...............good grief . The FO was , effectively, the COMMANDER of the aircraft and in an emergency can deviate from all rules and regs if he considers it necessary. Sadly, if the LHS pilot was determined to have lost his life, it is no longer an "Emergency" but a state of "Urgency". Full Pan call . In that state, if he felt uncomfortable taxying, inform ATC and comply with guidance. Looks to me like a very sad event very well handled.
In my day, all British manufactured aircraft could be taxiid from either seat. (There was a tiller on each side). Boeings could not. (Tiller only on the left). Of course, limited capability was available through differential breaking but you would not want to try that method for getting on the gate.
My concern is that yet again, so carefully monitored and, according to press reports, just having passed a Class one Medical, the pilot has a fatal heart attack ? Pretty useless screening, eh ?