Oh dear. This reminds me of some very painful experiences we had with my eldest at 5-6yrs old. A few times we bumped into a little boy she knew from school (and who we car pooled with sometimes) who was her friend at school (sometimes) but would be quite mean if he met her at a park or swimming pool and was there with other friends. We could not ever make her understand that following him around desperately trying to make him notice her and play with her was only making her more sad and him enjoy being mean even more.