What a wonderful friend you are.

I think there are a lot of good options.... a charm of some sort, or pendant.... I also really like the idea of a flowering plant.

My DP built me a beautiful white garden bed (for our miscarried child) and we filled it with a plant of which the flowers resemble burning candles. That garden has brought me much comfort.

I agree the fact that you acknowledge her baby will mean the world to her, and will most likely be enough. Keep it simple, she will be touched no matter what you do. I don't believe there is any mark to be overstepped here.

You're very brave and strong to be supporting her through this. I wish more people were like that.