Believe none of what they say, and only half of what you see.
Here's what you know. He stopped calling you 5-10 times a day and started calling her 5-10 times a day. That is not appropriate behavior for a married person, regardless of whether it went any further.
You don't know who she lives with, if anyone, and it doesn't matter whether she's lesbian or not. She could have cheated on the live-in girlfriend as easily as the live-in boyfriend. This isn't about her.
On the other hand, it could be a lot worse.
He stopped calling her when you exposed the relationship. She called him ONCE after that? (Was he supposed to change his number?) For all you know, she called to tell him to go back to you.
He moved out for two days, and now he's back. It sounds like things are unsettled, he's not happy, but he's not making any sudden moves.
You two got together very young, and often after a few years people start to wonder if the grass is greener. There's a reason it's called the Seven-Year Itch. If you find ways to spice up your life together, he may be less likely to stray.
