How do you deal with baby momma drama?

My boyfriend has tried to be friends with his daughters mother, but she always tries to come between us, she sends him dirty text messages all the time. (he says it's harmless flirting) and calls him when she's having problems with her man. how can i put a stop to this before it drives me insane? I love him and I don't want this to come between us . Everytime I try to talk to him about it he says I'm overreacting and that she's his baby's mother and I should stay out of it. please help