I would say, start with a trip to the doctor to see if there are any problems first.
Start with walking, and gradually improve the speed and distance of the walk. This needs to be a life style change and not just a diet or not just exercise. If they want to lose fat, they will need to seriously watch what they eat and change their eating habits, and exercise in some form most every day. In the beginning, they should be able to exercise to some extent every single day.
In agreeing with the others, does the person have a bicycle back ground at all. Did they ride bikes much when they were younger? There is no reason they couldn't ride a bicycle, but we don't know the person either and there may be a good reason they can't.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun