This form of question seems no less likely to raise problems than asking about the topic itself.

I'd suggest something more along the lines of "This is pretty standard advice, and it works for most people, but it's built on some assumptions about an average diet. How much should I be tailoring it for you?" Which is basically an indirect request for a status report, without implying anything about whether or not her current eating pattern is unhealthy. From that response, you can probably gauge to what degree you can safely bring it up directly.