|There are plenty of people who think the world is interested in unsolicited advice, while the truth is that they are the ones who need advice, namely to mind their own business. Of cours, if your daughter says this to her meddling co-worker, she will end up looking like the troublemaker.|
It is also tempting to follow an assertiveness model and something like, "I do not appreciate your comment. I find it to be hurtful, and I do not want you telling me how to eat." The problem with this approach is that it has a rather hostile flavor and is best used if other strategies fail.
The next time your daughter's co-worker proffers up another "certainly not low fat" comment, your daughter should respond with, "You're absolutely right," and then just look at her co-worker and say nothing else. The next words will have to come from the co-worker, and she is likely to respond by either moving awkwardly onto another subject or walking away, either of which is not an altogether bad outcome.
If this co-worker still tries to conduct a dietary discussion with your daughter, then your daughter should play the assertiveness card noted above. One way or another, this co-worker needs to understand that her "certainly not low fat" comment is certainly not acceptable.