Lying about something so serious as being pregnant means something else is going on. It's never the right thing to do! Does she lie about everything? How long has he known her? How old are they? Does she just feel he is not into her so she was testing him? Has she caught him in lies or knows things he's doing so she wanted to see who or what was more important?
Obviously, more is going on. People just don't go doing that, and there's other ways to ask a question about where you stand with someone. If he is not being open and the communication isn't there, lies are going to keep coming if she is so into him and can't gain some self respect and move on with her life, like most would have done. I don't know if she is outright crazy, or he has shown deceit in some way. Guys need to be honest with their feelings.
They only worked it out because of a few reasons. He wasn't treating her with respect, OR he was guilty about things he has been doing, OR he just doesn't care that much about her. It's easier to "work it out" for some guys, when you don't care, then deal with it. When something better comes along, then it's easier. Hopefully, she doesn't have a problem and he was doing something that made her insecure and they both learned a lesson. Since It must be humiliating to lie about something that serious. You wouldn't if things were going smoothly so the question is, is she crazy or what's the real truth that made her temp crazy?
I would talk to him. See if he's really into her, and if he isn't (since you know she is) tell him to let her go, even if it's hard to do, at least he is doing the right thing. It's not fair to her, and on the other hand, if he's not and she's that into him, you might really turn out to be grandma.
Oops, I meshed this question with WildIris response about her son.