Acknowledge/Accept/Allow: That this has happened and it might change your relationship.
Speak aloud their child's name: I can remember the first time a friend said "Stella"...it was a very odd experience, I'm even sure I was saying it/the pregnancy...now I love it, and I will never forget that time.
Listen, Don't change the subject quickly, and it's not time to fix anything or give out advice. For me, ask more questions or how I feel...cause I'm probably not going to tell you and I'm going to change subjects and you won't even realize it.
Mark the Dates, and remember. June 7th Stella died, Oct. 14 was to be her due date. Oct 15th is the wave of light day where you light a candle from 7pm-8pm...I think this dates stays the same year to year.
Pretty much anything you think is okay or would be nice to say, isn't. It's kinda weird. The best phrase is
"I don't know what to say" ....because neither do I.