I agree with countrigal. He's only asking you to sign an affidavit stating you saw him fall, which you did. If it goes any further, you are within your rights to amend said affidavit before signing it. If you are uncomfortable with the whole thing, just tell him you'd rather not get involved. And your supervisor is only thinking about the company, not your discomfort. I've seen this situation more than once, including one where the supervisor was the one asked to sign the document and he lied about the incident to make himself look better to the company. Not saying your supervisor would lie, just that she would most likely prefer you not get involved for reasons other than your discomfort.

Lioness, Queen of the Jungle