Excellent question! First, allow them to lead. If they want to talk about their therapeutic journey, be curious but respectful, asking/talking when appropriate. Therapy is deeply personal, so they may not want to share all the details. If you create a safe safe by telling them how much you love them and are there for them, that is enough.
It took something within them to open up about their experience. Honor them by listening actively and with your whole heart. Be the friend you’d want if you were in their shoes.