A simple “How are you doing today?” is enough. When you take the initiative to touch base, it tells her that you care about her feelings, and it also opens the door to opportunities to help. Sending a message that doesn’t warrant a reply is also nice. A text that says, “I’m thinking about you; just wanted you to know that I’m here,” is a way to show that you care, and your friend won’t feel obligated to reply if she’s having a bad day and doesn’t feel like talking.