Possible dumb question: Did she willingly disclose to everyone that that is how she lost the weight?
If so, then she doesn't strike me as a person who would be against someone coming to her and telling her about her body odor. Tactfully, that is.
A basic exchange, such as the following example, may be helpful:
You: Hi ______. You look nice today! Boy, you look like a new person. How do you feel having lost _____ lb?
Aromatic woman: Thanks! I feel great. I have so much more energy now.
You: Great, glad to hear it. Listen, I don't know if you're aware or not, but there's been a recent study about adequately maintaining personal hygiene after having one of those operations. Have you heard about that?
Ok, I know that may draw a fine line btwn genuine concern and lawsuit!