Here's my experience...
Got an absolutely brutal respritory infection 9 months ago - was not able to really exercise regularly for three months. I am just now getting back to normal fitness, so my gut is 5-6 months. I had the knowledge and structure to become fit, so someone who doesnt have that might take a little longer.
What's helpful is what you are already doing - setting small achievable goals. Strava or Wahoo fitness make it easier to track these goals.
The key is supporting the person, and encourangement. Make it fun, and make it enjoyable!!