You should find out how fast you can run and get your friend a computer for her bike so she can find the right gear to match your speed and then you can work from there.
I was looking to find out how fast the average human can run but since we're all different I got a wide range of results... I know that when I was a serious runner I could cover a 10 km route (6.3 miles) in 36 minutes which I think is is an average of roughly 10 mph / 16 kmh. This was considered to be quite good.
Your friend is probably running too high a gear as that relates well to the gearing I have on my fixed gear mtb (38 and 16) and when I'm taking it easy I can roll along at 25 kmh which is the speed of Olympic runners.