Use the child's name, and don't be afraid to talk about her.
Don't offer any clichés (she's in a better place, she's looking down on you, she was too precious for this world).
Its ok to say to her that it's shit, and you don't have words to make her feel better.
Offer practical support, not "if there's anything...." but I'll look after your dc while you attend to X, I will cook for you tonight etc.
Listen.
Don't compare her loss to anything you have been through unless you have also had a child who died.
Remember anniversaries and birthdays and always send a text, email, card or anything using her daughters name on that day.
Don't expect her to be over this in a month, a year or 5 years. Many will, and will drift away once they realise she isn't over this fast enough.
Don't trauma dump on her so she ends up comforting you, you can get support from people in your family or friendship group who don't know her.
The fact your asking is a huge positive step in supporting her 