Rules check

I was playing a competition yesterday and I was on a par 5 on my home course.

We're putting out and the flag is on the ground. Next thing a ball rolls up the green and hits the flag stopping it a few feet from the hole, much closer than it would have normally.

Is there a penalty, if so on who?

To clarify the guy behind us is a friend and we weren't mad he hit up, id say he's never reached this green in two before. Everyone was surprised.