Hi J.:
First,there is no reason to end a perfect friendship,because your toddlers are in a awkward learning stage. Talking to the childs mother, may help, then again,she may wonder why you waited so long to handle a problem,that should have been addressed imediately. Which ever adult is present during the confrontation,needs to handle it right away. Technically,A child this age,can't be a bully. Children this age,still don't fully understand their emotions or anyone elses.Toddlers have a limited understanding of how their behavior effects others.The child needs to know, how your daughter felt when she got slapped or bit. "That hurt(Mary)" "and you made her feel bad" "I know its hard to share,but this is not the right thing to do" Asking a child (why) they did something wrong,implies that there may be times when its (Okay) to be mean.Thats not to say you shouldn't look for a cause.If the reason for the aggression was a lack of sharing,then you need to (teach) them (how) to share.You need to (show them,) there are (better) ways to communicate unfairness or feelings of being upset.Take the time, to teach them problem solving skills.Make it fun. Sit on the floor with them both,and use imaginary play,to help both girls learn positive ways to resolve their differences.You could pretend to be the other child. Have your daughters friend play with a toy,and take it from her.Then explain to her how she asks for the toy back nicely. say "I was playing with that toy, please give it back" Then hand it back to her. Have her try it,then your daughter.Have them try it together. In a matter of a few minutes,you could resolve this issue.Tell them,if they ever have a problem sharing to ask an adult for help.In a short time, those girls will be very sweet to one another.J.,its so simple.They just need to be taught these skills,like anyone else. We can't expect them to automatically know everything that took us years to develope and learn. I wish you and your darlin daughter the best. J.