Just came across a suggested edit for this answer, which would fit better as a comment. Is there a way to convert the edit to a comment, while leaving the original answer alone?
No there isn't.
Reject the edit with the "should be a comment reason".
If it was a registered user they'd see the rejection and hopefully act on it.
As it's an anonymous edit all you can do is post it as a comment yourself (if you think it's valid).