Well, ideally, 1/4 wavelength, which isn't going to happen, so generally you make them as long as you can.
If you can get to the underside of the roof, you can use adhesive backed foil tape, often used in HVAC systems (not duct/duck tape…
Or, run 3 or 4 small gauge wire down to the body in each corner.
Personally, I'd be concerned on a couple of aspects…
-Mounting a steel plate to the roof to hold the magnetic mount antenna is either going to require some strong adhesives, or drilling holes. I'd suggest if hole drilling is in her future, just go straight for a thick mount NMO and put a decent NMO base antenna up there. It'll likely seal better.
-Steel is also going to rust and leave streaks, so you will need to protect it really well.
-She still needs to get the cable inside to the radio….
Or just find a half wave type antenna and skip the ground plane.