I am a bereaved parent, both my son and daughter died.
Firstly, and I say this kindly, please don't say to your friend anything about it being your worst nightmare, or anything that centres your feelings.
Lots of people do this to bereaved parents, and it puts a lot of pressure on us, are we supposed to sympathise with you, or apologise to you. I get people not being able to grasp the magnitude of such a loss, but the bereaved parent isn't the person to discuss that with.
Also don't try and say anything to make it 'better'. No "s/he's in a better place" or "at least you had them for X years" or anything similar. The best you can do is acknowledge its absolutely shit and there's nothing that will take their pain away. People often want to try and 'fix' things, but this is unfixable.
Message often, but don't require a response, don't get offended if she doesn't reply for a few days, just keep on being there. Not always about her loss either, general chit chat or however you usually communicate is fine.
If you're offering help then be direct, don't say " if you need anything....", ask if you can get them food, or do their shopping or laundry or whatever help you feel you can give.
Don't be offended if she backs away. After my children died I felt more able to be around acquaintances than friends, it's quite a common feeling, this caused great offence to many, and I lost a lot of friends.
Use her childs name, and don't talk about them in hushed tones or feel like it's better if you don't mention them. Also make a note of the date and make sure you acknowledge the birthday and this anniversary, again, using their name.
You're already showing how thoughtful you are by asking here, so I imagine youll be a source of great support💐