Stupid me , what should I do

So recently I saw these text messages in accident in my BFs phone , and this girl kept flirting with him saying baby boy and he was just giving her shirt answers until she said when I'm going to see you again  after she said that he stopped replying . She knows that I'm his girlfriend Bc she asked how I was doing in one of the text. Which gets so much more upset , what should I do should I let it be or should I message her saying stay away!!!