If you're going to do the whole, "wait until the first trimester is over to tell anyone including friends and family" thing to make sure that the pregnancy is viable thing, then you have some time before telling her. But when it's time to "go public" tell her first so that she doesn't hear it second hand. So that would mean waiting until the end of 12 weeks.
"Sarah, I have some news to share with you that I wanted to share before telling anyone else..."
I had the same situation with my best friend. She had been trying for over a year at least and couldn't understand why she was struggling with secondary infertility. She also had had a miscarriage. Luckily, while she was sad for herself because she wasn't the one making a pregnancy announcement, she was thrilled for me. Completely over the moon. Three months later she announced her own pregnancy and my middle daughter is three months older than her second child (also a daughter).