I've got to question the wisdom of a non company trained pilot being let in to help also. Sounds like he sat on his hands and didn't cause a problem. But he was not a known entity. What if it had been someone that thought they needed to assert themselves in a way that caused more of a distraction than a help? Suppose he started arguing with the F/O? What if he thought he knew "better" and tried to influence the F/O and caused a conflict? What if he was a pathological liar and wasn't really a pilot?
Get the Captain medical help, declare an emergency and land ASAP. Doesn't seem that hard to me.
Doesn't sound like you attended CRM
training.
You need to use all available resources.
Suppose the F/O had a heart attack!
You can't just sacrifice the entire aircraft
because you want to fly "Solo."