That's great that he finally told her. And OF COURSE it was the lie that was the worst part. Finding out someone you love and trusted has been abusing that trust is devastating, and makes you see them in a different light forever, even if you go through the healing and mending.
Having someone tell you something that they are afraid of (i.e. being poly), and being vulnerable knowing you may not accept them, but being honest anyway-- that is an awesome feeling, builds trust and makes relationships stronger-- EVEN if the information is not something you would want to hear.
I'm glad he's learned this lesson.
I'm curious though-- you say she's told all of her friends... are you sure she's done that, or maybe she's gone to a friend or two for support, seeing as she just found out that her BF was lying to her and having another relationship (and another CHILD with that relationship) without telling her? She may just need somebody to vent to and get support from.
And in the end the people who think he's wierd for being poly, or treat him differently, aren't people he wants to be friends with anyway. Maybe this will weed out the real friends from the acquaintances, which in the end is usually a very very good thing.