80-90 rpm is ideally were you should both be. I frequently go 90 to 100 rpm on my single.
You could try putting her on a spin bike or spin class to develop a faster cadence.
It is something that may not come naturally and has be trained to achieve which could take time and effort. I notice a similar thing when we ride our singles. I feel comfortable pushing against very light loads when standing, but my wife has to use a bigger gear when standing. Again its something that I developed over time and by riding with others that I learned more from just watching than actual instruction.