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Honey, any cheating that goes on is beyond the zodiac. If he's not happy, help him nip it at the buds. Respect should be mutual, regardlass of star sign. Some people are just bad eggs. If the relationship seems forced to you, imagine how your friend feels, all wrapped up and trying to make it work. Relationships are difficult and strenuous, and fights happen, but a relationship that takes more work than you get benefit has big red flags. A fight in front of a guest is a sign for help. I'm a Leo, and the usual trademark of a Leo is Loyalty. His partner doesn't sound very loyal, to me, and I'd be a bat out of hell before I'd give a cheater a second chance with me!!!
That's my opinion, anyways. Good luck to you and your friend.