Later in that Bell curve of cyclists thread Nate broke it down by age. Your partner is generally fit but untrained on the bike, so maybe find her age bracket and pick a W/kg a bit to the left of center. It’ll probably be close.
(FWIW, when I started, TR ballparked me at 1.0 W/kg. I was sedentary but not quite that sedentary, heh. 35-minute ramp, super confusing, thought it was broken.)