I would not recommend approaching about this specific topic. She knows she is underweight.
If she is a best friend then I take it you are in regular communication. On a regular basis, just start making comments about how unattractive you find unhealthy, women. Explain your values, repeatedly. Reference using BMI, the negative impact of airbrushed photos, impacts of anorexia, etc. etc.
As an example. What if you were thinking about getting breast implants. I recognize this is not the same as weight, but it is just an example. If your best friend started mentioning how it was against her values, that she lost respect for woman that found it necessary to get implants would it at least not give you pause, maybe question your choice? Or would you just say screw what my best friend thinks and it would have no impact on your thoughts?