A friend of mine lost his mom, and I asked him “how have things been for you?” he responded, “I miss her” What again can I respond to him?

You could say something compassionate and supportive, such as: "I'm really sorry to hear that. It's completely understandable to miss her. If you want to talk about her or anything you’re feeling, I'm here for you." This shows that you are listening and open to helping him process his feelings.