I would definately not just send an invitation. I like the idea of sending her a handwritten note, telling her your thinking of her, how sorry you are and dont be afraid to mention how precious her daughter was, that she was a very special baby, and you just want to be there for her in however she feels comfortable, whether that is avoiding baby talk for the next year, and just meeting for coffee without kids along, I think the 2 best things you could do is let her know youre there for her on her terms, and to validate the value of her baby's life. SOme poeple t hink if the baby died preterm or during birth it's not as much of a loss, so affirming to her what a special angel her baby is, will mean alot. You could make a little card, with an angel and something like her name and a pink bow and even write something like you are thinking of her and her little angel. Then, if she wants to talk more about her baby, she will know you are a safe person to go to, and that you will not avoid talking about it, which could be important to her after a time.