DD aged 10 is having some friendship issues. She had a best friend who moved away when she was 7. She then became friends with two girls A&B who were already best friends and their mums are very good friends. This seemed to work for a long while but of late, it seems she is constantly left out of games. The two girls always choose eachother if they need a partner, they will be at an event and then run off to be together leaving DD on her own. If one can't make it to something, (playdate) DD will get asked instead. A will say to her, B couldn't come. They give eachother expensive birthday gifts in front of DD, and get DD a craft kit (I know this is as much about the mums as anything but it is still upsetting).
One of the girls is a show off about her stuff and the things she can do - from musical instruments she plays to gymnastics and her holidays. They are both currently talking about their new bedrooms due to lofts being done out - DD shares with her sister.
When they leave her out, DD then gets sad, she says why, they try and cheer her up, nothing changes and then the events are repeated.
I have suggested:
She play with other girls in her class but she says there is nobody else in the class she wants to play with.
She write to her former best friend (we are still in touch) and re-establish that relationship
She message her 17 year lovely cousin for advice who she has become close to
We have played the glad game - listing things to be glad about but she was so glum she found it hard to be glad. 
I have said that she needs to tell them again how upsetting it is.
I want to speak to the mums concerned but DD thinks if I get involved they won't be friends with her anymore.
I have been here myself so this is all too familiar.
Any thoughts or help, please?