What should I tell my friend who just had a stillborn?

That you love her; that you care how she feels. If she has other children, you can offer to take them out. Bring her something that she loves. In general, just support her. Listen to her talk; sit with her in silence - whatever she needs.  

What you shouldn’t do is tell her that it was meant to be; that the baby would have been ill; that Hashem only gives us what we can handle; that she’ll have another baby.