Poor baby... they're so very very tiny and perfect then...
I'm so sorry for your friend's loss.
If you send a card, personalize it. Write a note in there. Tell her how heartbroken you are for her, that you love her and are there for her, no matter what, that you won't forget her or her son, now or a year from now or ten.
Some memento or memory-token would be nice. A necklace, pin, pendant. Something tangible she could wear close to her heart.