Is it cheating if you catfished?

So my cousin created a fake acct, and messaged her husband on facebook. He responded back, and flirted with her back. Theyre going to exchange nude pics too. Is this considered as cheating? He doesnt know its her, he thinks its actually the woman my cousin used as her display picture. Shes beyond hurt and is going to leave him. And shes only doing it cause she felt hes been cheating on her.