This married woman keeps flirting with my boyfriend. What do I do?

My boyfriend is a server at a restaurant. There is this attractive woman who is also a server there. He is only 22 and she is 10 years older and married with a child. When he started working there, I met her, and she was very nice to me. There were a couple of occasions where she said she might come meet up with us downtown some nights, but never came. He says he is just being nice, and he is like that with everyone he works with. He just wants to have friends at his job. He admits that she can be flirty with him. I was looking at his phone, (I know, I was being a snoop) and I uncovered a text message he sent her saying that he was out with the other people from work and it was boring without her. Then she texts him back saying all this flirty stuff. Then she sends him a picture of a heart saying "I like u" on it. He didn't respond.

I was so taken back by this. I first flipped out on him. I actually broke up with him and punched him in the face. He cried forever and eventually convinced me to be back with him, saying he never intended on hurting me and I was blowing it out of proportion. He promised me he would ignore her and he wouldn't go to the bar when she is there. I am still mad at him though. But tonight he has to work with her again. I am afraid of her flirting with him again. I actually managed to get her number from his phone, and I want to text her asking her to stop or else I will tell her husband.

What would you do?