My opinion, but arrived at through observation of a similar problem, and how it affected those concerned. I would tell the crew member that I was going to phone ops to get them replaced, and why, and that if I was proved wrong subsequently, then they would be at liberty to take whatever action they thought appropriate. I would also phone my union immediately afterwards.
I have every sympathy with those who suggest that you should encourage the individual to make their own excuses, but having seen someone trying to do that, everyone got hurt, not just the one who was unfit.