As far as I'm concerned, you can't win in this situation.

Five years ago I had cancer. I was reading everything I could find about it online. One of the things I read was by a woman who had lost a lot of weight because she had terminal cancer. She said several people told her how great she looked. When they asked what she was doing, she replied, "I'm dying of cancer."

Around that time I went to a party with some friends I hadn't seen in a few months. One of them appeared to be thinner than the last time I saw her. I honestly didn't know what to do. Tell her she looked good? Ask her what she'd been doing? I know it sounds stupid, and I'm pretty sure that if she had cancer I would know, but nonetheless I was stymied. I said nothing.

A few days later she called me, very hurt that she had worked hard to lose weight and "nobody said anything!"