Do i say something to the girl thats flirting with my bf?

Ive seen convosations between my boyfriend and this girl which really upset me. One was her saying i wish you were here giving me snuggles. My boyfriend says he goes along with it to shut her up. We very nearly broke up yesterday coz of this. Shall i message the girl to say something like leave my boyfriend alone, go find someone else? Coz if my bf cant say this to her i definately will. Not sure if thats a good idea though. Any input would be much appreciated :) x