Girls being sly in relationships?

So I'm friends with this couple and they've been together for quite a long time (I think about two years at the time). She started to facebook message and text this other guy a lot, told me about him and mentioned a lot about him in front of her boyfriend. To be honest the boyfriend can't really do subtle - anyone could read his facial reactions, he was uncomfortable. I assumed from the texts etc that I'd seen that this other guy liked her although I never voiced this and she denied it a little. Eventually some months later her boyfriend actually confronted her about it. He had bottled it up so much and had only ever mentioned it to one friend so far as I'm aware. Now I don't think she would have done anything with the guy, but is it normal for girls in relationships to string along a guy who clearly likes them despite the fact that they're in a supposedly happy relationship Sorry that it's a long story!:$

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