I am at my wits end and could use some advise/perspective. Sorry this a bit of a long one - don't want to drip feed.
DD 8 is in year 3 and has been part of a same set of friends - about 6 girls - since nursery. We all live quite close to each other and occasionally meet up socially with family etc. There are varying levels of friendships between parents in the group as well. One of the girls in the group pf 6 has always been bossy and dictatorial. She gave DD a tough time when DD first joined the nursery but it was all handled then and things seemed to settle down a bit. We put down any spats that came up as normal behaviour of kids learning to socialise.
Year 2 DD and the girl had multiple issues and it was clear DD was being constantly picked on. The class teacher called me once when the child hit DD in the playground. The school made a note of the incident and kept a close eye and I know worked with DD as well reinteracting the importance of letting teachers know fo such behaviour. After that DD was excluded from 2 birthday parties in the group (the two parents are close ) and DD was told in the playground that it was because she complained about this child's behaviour. We helped DD understand that but it has made her more reluctant to flag issues.
Year 3 they are in different classes but its started again she constantly tells DD she is "stupid", "not likeable" etc and once to "go shoot herself" . DD is also adopted (we have had her since she was born) and DD has been open about that at school, there were a few mean things said by this child around that as well - which again was promptly addressed by the school.
Now DD has always lacked confidence and this has hit her hard. I encourage her to walk away but the problem is she still wants to play with the other 4 girls . While a few other parents have told me they have had issues with this child, she seems to be giving my DD a particularly difficult time.
So, help me. How do I teach DD to rise above such behaviour? I know she will face such people everywhere in life but I am struggling to be able to teach her to not care about what they say. I get it is a life skill but its breaking my heart to see her so sad and starting to put herself down.
One of the other Mums thinks I should raise this with the childs Mum. I think the Mum is quite aware of her DD's personality (the Mum is lovely btw) and since the school have raised it with the parents, I don't see what me raising it with the Mum would achieve?
I have had a rough time this year emotionally so willing to be told I am not handling this well/ at all. WWYD ? All help gratefully received. DD has just cried herself to sleep after today's episode. I should also mention DD wants to continue being BFFs with this child 
- help me teach a 8 year old to recognize toxic friendships.