Is flirting cheating?

Taylor • You were given this life because you are strong enough to live it
My boyfriend's friend came over to my house, asking for advice. His girlfriend told him to text one of her friends back for her while she was getting ready. When he opened her messages the conversation that he was brought to was to a guy. She seemed flirty in the texts saying "I'm just enticing you ;)" to the guy. The next text was "oh what a shame. I didn't do a good enough job then huh? ;)" 
That's all that was in the text conversation with him. She had to of deleted the texts from him and the previous ones. He ignored it and just texted her friend back. 
He then came over and wanted to know if flirting was cheating. I believe it can be but I want to know your opinions. 

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