My ex has another child and he told me very early on in their pregnancy. I could've killed him because they were together for such a short time (3-4mo), their relationship sucked & he bltched about her to me incessantly & had told me only a month or so before that he had decided to get the snip as he didn't see our child very often let alone adding another. If she had told me whether by email or phone I'm not sure what I'd have said but I am glad it came from ex. I was upset at the time because he is irresponsible and an average father, whereas DF & I were actually planning a child & getting married etc but he gave her a sibling before I did, so the 'FINALLY' thing would bother me.
Fast forward 18mo & I have a 6mo - I told ex and would never expect DF to do it, even if I were on bad terms with ex.
It sounds like you know which way you should go, so congratulations and good luck. I do hope she doesn't take it badly :-)