My eldest daughter had an imaginary friend for many years, so my advice is don’t worry too much about it, this friend will disappear as she gets older. As Rosie seems to be the tormentor, use the opportunity to talk with your daughter about why she thinks Rosie does these things, you may find that there is a bigger problem such as the child is experiencing this at child care, pre-school etc, and in this case, mentioning it to the adult in charge might be needed.

When my kids had their imaginary friends, I just played along. My daughter wrote whole books about her friends. She’s 13 now and the imaginary friends have gone since years. My son never had imaginary friends.

An imaginary friend is very real to the child. If they can see you are taking them seriously it helps them deal with things better.
Get the child to write down how it makes her feel when the ‘friend’ is nasty to her and get your granddaughter to show her .
This will start a dialogue with your granddaughter about bullying and might help her in the real world too
Good luck , It can be a worry but i’m sure she will grow out of it soon enough .

I used to write a letter to my daughter’s imaginary friend and she showed it to her every time she’s around. That way that she will know how I feel about her. Kind words and understanding both of them. Sometimes their attitude to each other ca create a problem, that’s when I told my daughter to tell her friend to have a “time out.” It works at times. Now that she’s grown, she’s no longer there.

Imaginary friends are usually outgrown. I would play along and treat Rosie as a real person. Start to worry if it causes problems or goes on for too long