Need help with my prego BFF or better said she needs it?

Um, I'd say let her know because that's kind of important and may weigh in her decisions. I found out my bf had been cheating on me while I was pregnant, but I found out only 2 weeks before my son. I twas a horrible time to find out, and worse I had been building up these ideas of how things would be when I had the baby. If you're really good friends you need to let her know, she will hate you if she finds out you knew and never told her.