He.

Is.

Married.

Nothing more need be said.

It doesn't matter if he's "happily married" or if "he wants to get out of it."

He's married. He's lying to her and his wife, and he's cheating on his wife. He's also cheating on the familiy he should be nurturing with his wife. Even if he's not actually having sex with your friend, he's cheating emotionally.

There are no kind words. There's no gentle advice. She needs an in-her-face intervention, IME. No lasting happiness will ever come from being party to the breakup of a marriage. Even if he leaves his current wife and marries her, she'll never be able to fully trust him.

He's a liar and a cheater. Remember, cheaters cheat. If he cheats once, he'll cheat again.