Citric acid lowers the pH, which can easily interfere with the reagent. Any other acid that will increase solubility of lime will have exactly the same effect.
Low pH and presence of fluorides makes a solution equivalent to solution of hydrofluoric acid (doesn't matter where the ions come from), so there is a grain of truth to what your friend told you. However, at concentrations you are working with I wouldn't worry much.
Sadly, can't help much more, idiots at hannainst site decided to redirect me to my national site without asking me whether I wan't it, or not, so I don't see your page, just the main page
Perhaps someone located in US will have more luck.