I would strongly suggest two important things. 1) that she learn to not compare herself to anyone else. She'll progress at her rate. Period. Comparisons are counter productive. 2) Get a heart rate monitor and learn to use it to gauge your effort so she can work up to three or four half-hour rides at a reasonable aerobic pace that doesn't leave her gasping for air but gets her heart rate up to an appropriate level. For somebody out of shape that can be a very low number.
If she begins to keep records of distance, heart rate, time etc she will continue to see progress every week and ever month. My story is not unlike hers. A year and a half ago I was 280 and out of wind at a heart rate of 130 for ten minutes. My first ride was about 3 miles. This year I have rolled 4,000mi so far with two century rides and 60 miles is a piece of cake. It's a slow but steady process if you keep at it.