Having somebody extra on the flight deck is worthwhile imho. Personally I would use them as an extra pair of hands, memory aide .... get me this book, remind me to do this, hold this checklist out, read this to me ..... and most importantly look after my mate who has keeled over.
Who should this person be? My airline suggests cabin crew member, whilst they may not be familiar with specific manuals, they are taught to read checklists (in theory knowing to stop when we raise a hand etc) and are probably better at tending our stricken colleague than a ppl holder. On balance I would favour the latter but probably tend to manage the flight myself with the maximum help from George.