Ok I had a similar issue but my friend had the biting child.. She tried to make her sit out.. Or would put her inside (if we were outside) yelled.. nothing worked..

In the end. she would pick up my DD and comfort her and I would tell her DD off (in a nice way) I said I don't like you biting Olivia. It is mean. (they were 2) So a little older then your DD but I think she would understand..

OR. Maybe when she bites hand her a toy (maybe buy a rubber duck or something) so when she bites say No biting your friends bite this.

unfort it is a communication thing and she is at an age where she can't communicate exactly what she wants or what the issue is..

Is it only when you are at their house?? or is it the same if they visit you? I found my friends DD wouldn't bite or hit (as much) when they visited my house. So when I went to her house I took some of DD's toys with me and things settled down

hope it all stops for you soon