Is it cruel for a man who knows his female friend likes him to continue flirting with her even while he has a girlfriend?

Ie. He stares at her intensely when he sees her, he does things to get her attention, he stares at her breasts etc. I mean if he wanted to be just friends then behave like one and don’t blur the lines right? This isn’t some random female. He’s known her for a year.
Is it cruel for a man who knows his female friend likes him to continue flirting with her even while he has a girlfriend?
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