45yr old female had 3b colon cancer. Now clear for 1 yr. She refuses to diet/exercise. Obese. Odds this will make cancer return?

Very low. Diet, EXERCIZE and general good health is a benefit! Not a hazard.
Not clear. There is data in breast cancer patients that post treatment exercise leads to less recurrences, but a bit less clear in colon cancer. There is data to suggest that post treatment exercise colon cancer patients liver longer though. And exercise is wise, in general, for overall health. Best of luck.