Firstly if she is working and has been working at least 4 years, she is a woman. Not a girl. You are using this as a way to talk down about her.
Secondly, how in years of her working for him has he never addressed her attitude. I am guessing he hasnt. Even before he was with you, he shouldnt have excepted this.
The truth is, is that your issue is with him. He, on some level, likes it. He accepted it before you got together, so probably feels weird about tackling it when he has been ok for so long. She will know he is tackling it because of you.
But he has accepted her behaviour. So she thinks it's fine and acceptable.
I dont work with my partner. But if a colleague tried to cut him out of a converstation at a works party, they would know its not acceptable.
I have never let anyone that works for me email or contact me in a over familiar manner either. They have been pulled if I dont like the way they act.
Yes, men have flirted at work and they are cut short.
If he was handling this in a professional manner there wouldn't be an issue.
And to be fair. She may not fancy him. I know loads of people who think flirting with their boss, keeps the boss sweet and in their pocket. I have had a couple of men, try it with me.
She may have been cold towards you because she felt you would threaten the working relationship she has with him. Rather than actually want to date him, herself.