I used a dolly for 3 years while I had a car that could not be flat towed. Then I bought a new car that can be flat towed, and flat towed for 7 years. I would do either one again if the need arose, but flat towing is, hands down, my preference. Given the option, I would not do anything but flat tow.
If you are thinking of replacing her car in the next couple of years, and you will be getting a car that can be flat towed when you do, it might make financial sense to go ahead and replace her car now, and set the new car up for flat towing. Buying a dolly is $2,000 to $5,000. If you do flat tow in your future, you would be lucky to get half of that back on selling the dolly.