He broke your trust. She wasn't there when you needed her most. He said he'd never do that. She did the unthinkable.
An argument stems from a place of hurt, anger, & deception. Once you say those hurtful words, you can't take them back. I don't blame you for being mad, but hear me out!
"Good sense and discretion make a man slow to anger, And it is his honor and glory to overlook a transgression or an offense [without seeking revenge and harboring resentment]." Proverbs 19:11 AMP
Words have a big impact now more than ever. In the heat of the moment, it's easy to feel like you've got to let them have it and go toe to toe. Don't do that. You're never going to resolve your issues as long as there's a challenging match and the game is tied. Listening is a great start. Don't voice the issues outside of the person you have them with. Keep a calm head and you could see way different results. At the end of the day, we all want to feel hear and understood.
"Let no one deceive you with empty arguments [that encourage you to sin], for because of these things the wrath of God comes upon the sons of disobedience [those who habitually sin]." Ephesians 5: 6 AMP
Your homegirl never liked him in the first place. The guy at work thinks he can treat you better. Your mom has always had a bad feeling about her. You're going to a lot of opinions in the midst of a bad moment between you and your partner. Some might give you advice worth taking and others will give you an opportunity to get even no matter the cost. In these times remember your morals. I know it may be hard to think of how you could even care about that person, but at some point in time, YOU WERE HEAD OVER HEELS. You can't lie to me lol.
Arguments are going to happen and people are going to have opinions IF you allow them the access to. What happens in the relationship needs to stay in the relationship. Period.