First and foremost: be a good listener. Many times people just need to vent, and to be able to do so without feeling judged.
Next, be supportive of the diet and lifestyle changes that your friend is trying to make. If your friend is trying to go for a walk every day, offer to join them. If your friend is coming over for dinner, ask if there is anything special you can purchase or make to help them stay on track with their diet.
Response from Kelly
You are a good friend to want to offer your support. In addition to your friend speaking with their doctor, these articles may be helpful: