Everyone is different. Some people would be comforted by talking about the baby, all day long, and feeling good knowing that others remember him. For others, any mention of him will be simply to painful to bear, and will unleash waves of regret about the things they never got to do together.
Is she in the same area you are? If she is, I would call her or send her a note about a week in advance, saying, "Hey, I've been thinking about you. Would you like to go to lunch on the 27th?" If she says, "Yes, that would be great," you'll know that your presence will comfort her. If she says, "Thanks, but no," take that as a sign that being around others would be too exhausting and painful for her.
If she's not local, send her an e-mail, e-card or Facebook post on that day that says, "Thinking about you! Love you a lot!" That way, she'll know you thought about her (and the baby), she'll know you care, but she won't have to talk about the baby unless she wants to. She may just e-mail back or say nothing, or she may phone you, but it will be up to her. She'll know best what she needs.
Bless you for caring!