At 12 volts the portable uses 6.5 amps.
Momma!! That's a LOT!!! You're not going to like packing those batteries around much better than packing around oxygen tanks.
The Inogen G4 consumes about 20 watts when it runs, and it can consume up to an additional 20+ watts if running while charging. That's 1.6 amps @12.5 volts just running, and a bit over 3.2 amps when running and charging, assuming a fast charge. That translates to half the battery weight for a given operation time.
Also the G4 car charger is just a cigarette lighter to 5.5/2.5mm connector, with no "brick" in the line.
Not my job to persuade you one way or the other, but compare those specs carefully before you plunk your money down.
If she needs more than 2-3 liters per minute then the really light compact units may be out of the running.
