I wouldn't worry about it. A similar thing happened to me once. I had a new boy start at my daycare. He was here maybe about 2 weeks when one of the other boys scratched him on the neck while playing. Dad freaked on me. I just told him that kids will be kids and things will happen. The other boy was disciplined accordingly and that is the best I could do. Needless to say they pulled him out of daycare a week later.

We can't watch children constantly all day long and even then kids can do things to other kids right in front of us. If you are in the process of working with the biting child to quit biting the other parents should be patient and understanding as hard as it may be, if it was their child doing it they would want the same from the other parents.

I say just ignore the whole situation. If dad brings it up again tell him you didn't see it happen and that you are working with the other child to curb the bad behaviour. If they pull their child then so be it.