I am surprised that this child was bitten so hard as to leave a mark and he didn't scream? Cry? try to tell?
Obviously it would depend on where it was, but could he have bitten himself?
In the future I would never tell a parent I didn't see anything - especially in a text where you are saying in writing that you were not supervising the children (my regs are clear that a child needs to be in my sight at all times). I do think mom was making a threat - but essentially her child has an injury and you are admitting you have no idea it was there. I might have said "Hmm, we had a great day of play yesterday. Billy did have an argument with a friend over a toy, but it didn't get physical. At no point did he scream/cry that he was hurt. I will certainly keep a close eye on him and get to the bottom of it." After that I might say "Gee mom I have been keeping an extra close eye on Billy. While I haven't noticed any other children biting, I have noticed Billy biting himself and others." (if that's indeed the case) But also in the future I would not keep serious issues from parents.