Just be sincere and express your feelings. Say something nice and maybe share a related memory.
The key is to be genuine. A simple 'That's a wonderful story!' or 'I'm so happy you shared this on your birthday' can go a long way. You could also ask a follow-up question to show your interest or share a similar experience you had.
Well, you could start by congratulating the person and showing your appreciation for sharing the story. Then, mention how it made you feel or what you liked about it. Maybe add a funny or touching comment based on the story.