Wow my heart breaks for her. I will certainly be praying for them, and for you as she will need you very much. I do disagree, however, about asking what type of service she would like for her son. When I was pregnant with our triplets, I knew once my water broke that they were too tiny to survive. The LAST thing I wanted to hear was someone asking about the funeral. We dealt with that when it happened.
I suggest that there are no words except "I'm sorry". She doesn't need to hear anything else. Food is good, cleaning her house, maybe. Taking care of her other children, mowing her yard, going to the cemetary with her in the next few months...
I think the one thing I needed most was for people to let me talk about my children. I think sometimes we think it will hurt too bad to bring them up (and it will at first), but all she will have is her memories, and by you talking to her about him sometimes, it will let her know that you still love and care about him.
Take her to lunch, or a pedi/mani in the weeks after everything settles down. MAKE her get out of the house, because trust me, she isn't going to want to get out of the bed!
You are a good friend to care enough to want to do/say the right things. God bless you for being there for her! Please keep us updated.