First, make sure your friend is up to sharing more about their experience. ("Is it OK if I ask you about that?") They may be grateful for the opportunity to open up, or they might not want to get into it right now, and either way is fine. If you do ask more questions about their experience, show that you're coming from a place of care and curiosity. Make sure they know they don't have to answer or can stop the conversation at any point.