Is this girl acting like a slut or just friendly?

I work with a guy and a girl who has a boyfriend. I sit near the guy and notice a bunch of things between them. They seem to have banter all the time and she uses a flirty girl voice every time and they also flirt. They constantly joke around as well.
The last few times, he holding her hand and drawing on it with a marker and she was giggling.
Another time, he tied her hands with an elastic rope and was touching her face.

Is this ok behavior from a girl who has a boyfriend? Just friendly?
Is this girl acting like a slut or just friendly?
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