I'm going to tell my work friend his partner is cheating
What made you think your friend's girlfriend is cheating on him?
She lied about having lunch with work friends
Maybe it was just a mistake.  She could have been referring to another lunch date she had.  Besides, people have the right to go out with their friends if they want to.
Oh, I knew she was a lying skank
It sounds like you have already made up your mind about this woman, and maybe you have good reason to distrust her.  However, I would not try to break up their relationship based on so little evidence.  If you are concerned for your friend, let him know how you feel, but don't make direct accusations.