I'm in sort of the same situation. I have friend since high school many years ago that I haven't told for various reasons. I'm debating if I should even tell her at all. She's in a different state so she really doesn't see me, but I don't want to lie when she asks about my weight loss when she does see me.
I also work in an environment where we have potlucks and food for any reason. On top of that, I have a co-worker who always pushes food on others. I'm still torn on whether or not I will tell my co-workers that I'm having/had surgery.