'm assuming the CC member either can't find the flight crew oxygen masks or doesn't know how to use them.
This would be your first major plot hole, considering all cabin crew are instructed in the use of flight deck oxygen in the event it must be administered during pilot incapacitation events. You'd be better going down the road of oxygen line contamination - for example, oil was found in the O2 lines and that led to the incapacitation.
Also, if your character had spent an hour or two in the sim and read the FCOM, they would know where the Push-to-Talk (transmit) button is. Otherwise, that was a lot of wasted time and effort on the part of the character.
Finally, that much time in a simulator for a 500-hour pilot would be enough that they'd be able to hand fly the airplane, albeit not with the finesse of a more experienced pilot. It would be the automation that would be a problem, but then this could be resolved by discussing the matter over the radio, leaving the automatics on and having the thing autoland at an airport with CAT III capability.
Ultimately, can it be done? Yes.
Would it be safe? No.
Are injuries likely? Yes, but perhaps more from a subsequent evacuation than the landing. Now, put a non-pilot in place of your cabin crew member who is a pilot and the outcome is not as likely.