I don't see it as a permission to cheat as long as the 2 people in this type of relationship are aware about it and agree with it. This guy was honest with your friend and told her about his lifestyle from early on, which is a good thing. It's now just up to her to accept this type of relationship or move on.
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im in a plural marraige with my two wives working on a third. If a woman gives the milk for free why would any guy want to purchase them.
I don't dislike them but I dislike the concept in regards to myself personally. Meaning it doesn't suit what I'd want in a relationship but I don't care if other people want to partake in it. That's their life.
I know a girl who was seeing a guy who was in a poly relationship. She left him soon after because she was not ok with him seeing other people at the same time as her. I personally don't agree with them either, I'm completely happy just seeing one person at a time.
I don't think there's anything wrong with it as long as all people involved are aware of the nature of the relationship and are happy to be in it and feel that they're able to express any concerns or leave at any time.
It's polyamorous, and it doesn't just mean fucking multiple people and never marrying. It means having relationships involving more than one person. How that plays out is different for different people.
As for myself? I would not get into a relationship with someone who is poly because I'm not. I am monogamous so it wold not work.