Well, such a thing can't transfer many amps; it'd be a slow charge. Not only that but the fuse in the cig lighter, plus the drop in the typically small-ish gauge wiring is going to further hurt results.
If I really wanted to test: I'd remove both batteries and find an active load (or a good power supply) to simulate a 10V or so battery. Then apply 13.8V, and measure how much current goes through. I might go out on a limb and crank up the volts to see when I hit 10A also.
Want to say most car batteries are 40-50 amp-hours, so to bring up a battery to cranking state may require an hour plus of time.
Unfortunately none of that makes for a good review. Have her write something to the effect that installing this device in the trunk has given her confidence to stray far from home, knowing that, if ever stranded by a dead battery, she could jump start without having to touch the car hood. A normally very-complex jumping procedure is slashed to a trivial exercise for the non-carphile.