Hi Nikki
so sorry for your friend. Don't worry about keeping it together for her, she will appreciate you feeling a loss as well.
Don't say "It was for the best" "It was gods will" "I know what you are going through" (unless you do). But do be their when she needs a shoulder to cry on and please talk about her little boy, mention his name, how beautiful he is.
A memory box is a nice gift for her other child and her & DH to fill with congratulations/sympathy cards and pictures etc.
If you have not already buy a small gift to go with him, or for her to keep, ask about flowers at the funeral as some people have them some don't.
Don't assume her child will not be at the funeral, she must make a judgement call on this as to how her DD will handle it. My DD came to her brothers funeral as I felt it important for her to have closure and realise he was not coming back. I also cry in front of DD but explain why I am sad and how I miss Benjamin and how happy she makes me.
Tread carefully re the funeral she may wish to have only immediate family or she may want friends as well let her guide you but you may need to ask.
Also second the practical issues of eating you do not feel like it and cooking seems like such an effort. You feel like your world has ended and how can people go on doing normal things when this has happened. Don't they realise your baby is dead. Just be there for her and don't just say get in touch if you want something because it is unlikely she will. Suggest things like "I am popping to the shops I will pick you up xyz whilst I am there do you need anything else?" It really is difficult as what is right for some people is not for others. I loved having flowers some people don't.
Her due date will also be difficult, a phone call would probably be appreciated. Further down the line I feel his birthday is a day to remember rather than the day he died.
On a practical note she is entitled to claim child benefit and therefore her child trust fund which she may not realise or may not want initially. Only now do I wish I had claimed for Bens so that his twin could receive the benefit of it. Also many places charge a reduced rate for childrens funerals.
You can give her some websites which might be useful lots are listed on some of the other threads, sorry do not have chance to search.
The best advice I can give is talk to her and her DH and DD and guage what they want.
You sound like a caring friend and she is lucky to have you.
HTH Charley