Co-pilot gets sick, stewardess helps land airplane

Given this set of circumstances, what types of assistance would a
commercial-rated pilot be able to offer assuming she is not rated on
this type aircraft? I would guess-communications with ATC primarily.
What else?

She, as mentioned, can help run the radio. She also can read "check
lists."

The pilot knows where the various switches/levers are.

She can also call off speeds when landing.

Depending upon how much skill she has, she can double check whether the
pilot correctly did what the check list recommended. Indeed, that and the
radio are the only two areas where "flight" experience is useful.