When people talk about "cheating" they're talking about breaking the rules through deceit or fraud. Think about cheating in a game--it's breaking the rules in a way to try to avoid getting caught. Same thing with a marriage. So, it depends on what the rules are for that particular marriage. It can be what the two expressly agreed. But, I think there's also certain things that are generally understood though when it comes to marriage and marital vows to be "faithful" and similar vows. Most people consider monogamy implicitly part of the marriage rules. Maybe some of the other things Laz posted are a little more gray, but sex with someone else?--that's cheating in almost everyone's book.
Also, besides what the vast majority of Americans would say are the rules for marriage, how is your friend planning to go about it. Is he planning to do this openly or secretly without his wife's knowledge? Is he planning to lie if asked where he was? Is he going to use cash he's secreted away? Is he hiding his web browsing and communications through hobby email? I think the answers to those questions make it pretty clear that it's cheating. Of course if your friend has an understanding with his wife that they can have sex with others, then no cheating, but that doesn't sound like that's the case you describe.
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