I was recently walking by my friends grave when a very large black feather blew past me and onto the grave stone. It was very very large and over sized which could indicate that it couldn't have been from a crow. Could this mean anything as it may have implications?

Implications for what? So far, I see no hidden meaning.

Yesterday when you weren't there, a small yellow leaf may have fallen on the grave itself. Significance? Yellow was the friend's favorite color?

When you see something that appears connected and significant, it's connected and significant to you alone.
If you need to find meaning in it, do so, and it may help open you to a part of yourself that was hidden or denied, such as a remark you wish you hadn't said, or a need to feel that your friend's spirit is OK.
There are people who make a living out of soothsaying, people who enjoy taking your money to tell you what they think it means.