Is it ok for a girl to flirt with a coworker all the time and not mention boyfriend while she's in a relationship?

I have a friend and I know her boyfriend. She told me she flirts with this guy at work all the time and even never mentions her boyfriend to that guy co-worker. I told her it's not normal and stupid but she's getting defensive and says there is nothing wrong with that.

Who is right?
Is it ok for a girl to flirt with a coworker all the time and not mention boyfriend while she's in a relationship?
Post Opinion