I found that distances on elliptical trainers and treadmills cannot easily be compared. They are sometimes not even the same between different manufacturers. If she really wants to equate the two, I would go about it a round about way. Check her heart rate on both machines to see whether the effort is about the same and then see what distances she covers for 10 minutes on either equipment. Then you will have an approximation how those different machine readings compare to one another.
Other than that, I second Harris’ and Paul’s comments (even though I take exception with the elliptical being for older people – good thing he only spoke for himself ;-).