Should I message her?

So I found out that this girl has been flirting with my boyfriend saying things like "your butt looks cute today" "I'm wearing white lace ;)" "you seem like you would give nice hugs" "I'm going to wear yoga pants today" and the thing is she KNOWS about me! She has even told my boyfriend that she is afraid of me. Well I got her number and I'm debating on if I should message her or not. I wouldn't flip out or curse or anything like that. I would just tell her that I know what she is doing and that she is a extremely disrespectful person and that it will stop. Is it a good idea or not? Is it childish? And just to make it clear my boyfriend will no longer speak to her.