Is my friend's proposal for an open relationship a good idea?

I have a friend and he is 23. He has not had much experience dating girls, but has had a serious relationship with a person whom is 32. Anyways he was caught cheating on her, they sort of broke up but are now they are looking into polyamory. Question is, he had asked her about it and she said to bring someone home to see if she is ok with it? He really thinks it would help them and she is long distance, while she only wants to be with him, even if this makes him depressed. Is this such a good idea?

1.) He was not "caught cheating on her," he cheated on her. Beware of using passive voice to let yourself or others off the hook for their own actions.

2.) It is never okay to "bring someone home to see" if polyamory works out. That person is another human being, with feelings and experiences. They are not an experiment. It is cruel to use them to test out your relationship. Dragging a third person into this is not appropriate. If there's a hill you choose to die on when talking to your friend, it should be this one.

3.) If "she only wants to be with him," then they shouldn't be polyamorous. If the terms of the relationship make him depressed, he shouldn't be in this relationship. Polyamory does not help solve problems with existing relationships and will not fix the issues with cheating, long-distance, or depression. 

This is not a good idea, but:

4.) None of this matters, because you're not the one making the call. Your friend is going to make his own choices, and it's very rare that someone listens to their friends when they tell them a certain relationship is a bad idea. You can advise and support your friend, but you can't control how and who he dates. Writing to a third-party advice columnist about a choice someone else is making will accomplish nothing but making you feel righteously frustrated. Try to disinvest yourself from this situation, don't take it personally when he ignores your advice, and get out of the drama splash zone.