Need advice plz

I’m gonna try to make this short, simple, and hopefully make sense. Okay, so my boyfriend has told me in the recently how this on specific girl keeps flirting with him over and over again and won’t stop. So last night while I was on the phone with him he told me that he’s supposed to be tutoring someone in math this weekend so I asked him who and he told me it was that girl that flirts with him. What should I do? I know I should say something but I don’t know how.