Just smile and thank them for thier opinion, then move on. The best comeback you can give is to simply lose weight and get better health. Maybe whenyour friend comes to you looking for support on the lates cold or flu bug she has, you can casually mention "Gee, with eating healthier I almost never get sick anymore"

The next time she wants to argue, ask her directly "Did you read the book?" She will probably say no. After that promise her you are more then willing to have a debate about it if she gets an informed opinion, not repeating the gossip she heard at work.