My little girl died totally unexpectedly almost 4 months ago. People were amazing - heartfelt cards, regular texts and emails really touched me, although phonecalls and visits were harder, and remain so. I would recommend just to continue to be there, just throughout. Friends who just made an initial email, and now are only calling me again, I almost resent... I feel I have to explain my grief journey, and make them feel good by saying I am coping... Those who consistently just said, "hi, thinking of you both and Mia" are much easier to approach.
If your friend is like me, she won't ask for help. She will know people want to, but they can't relieve the unbearable pain of loss. But if you can keep offering ideas or visits, she might agree. And keeping busy helps a bit.
And yes, please do acknowledge her beautiful boy. So important. She might cry, but that doesn't matter. She might laugh at special memories or photos you have. Letting her know that he existed, that he mattered, that he continues to live on in your memories, will mean a lot.
Your son might continue to talk about his friend. We love hearing these stories, where a little friend continues to play with Mia, taking her on adventures; another friend sent a big hug and kiss to the stars this morning when her name was mentioned... Do share anything like this, it will mean so much.