It sounds like you're describing a situation where your friend is inconsistent in their behavior or actions. To better understand and provide advice, I would need more context about what specific "one thing" your friend is doing and the circumstances surrounding it.

Some questions that could help clarify:

1. What is this "one thing" that your friend does?
2. Is there a pattern to when they do it or don't do it?
3. How does this inconsistency affect you or others?
4. Has your friend expressed any reasons for their inconsistent behavior?

With more details, I can offer more targeted suggestions on how to address the situation or discuss it with your friend. In general, open communication and understanding the underlying reasons for someone's behavior can often help in these types of situations.