DD's best friend (will call her Anna) is fundamentally a nice girl and they usually get on really well. True, Anna is the more dominant character - she usually decides what game they play etc, and usually gets first 'pick' of which character to be. But in many ways this isn't something DD minds, they play fabulous games and she's not the kind to get upset if she doesn't get her way.
They do also play a lot with another little girl, will call her Betsy, but I have been encouraging DD for ages to widen the friendship circle. The 3 of them are often falling out/quarrelling in minor ways, and I suspect they just need a break from the intensity of only playing with each other.
However a new aspect is that Anna seems to think she can pick up and 'drop' my DD whenever it suits her. And tbf, she's right - DD is that girl who will just feel upset and will wait patiently until Anna comes back and 'chooses' her again. :(
Yesterday DD was very upset because Anna had played with another girl (Clare) at playtime (all fine, I'm delighted if they're playing with others!!) but had then come over to DD to say 'I like Clare more than you!' And then proceeded to ignore every word my DD said to her for the rest of the day (turning her back on her etc)
I can totally understand that my DD will have irritated Anna all day by doing exactly what she does when she's worried - DD will have spent all day desperately trying to get Anna to reply to her which will have irritated Anna AND confirmed that she's the powerful one.
I had a chat with DD last night and briefly this morning and she did agree that she would play with someone else today 'but if Anna is nice to me I'll play with her'.

I pushed back against that and explained why she shouldn't do that!
I don't think it will have worked, it's happened before and I suspect DD will be delighted right now that Anna has decided to be nice to her again (or she'll be crying in the playground because she doesn't have anyone to play with...)
What can I do? I talk to DD about this stuff when it arises. I think I have good advice! But it's hard to get her to follow it!
How do I make her more assertive...?
I encourage other friendships btw, we have playdates - there are 5 or 6 other girls she really likes and would definitely call her friends. But if Anna wants to play with her today she probably won't have taken up playing with any others.
I can see this being a long-term problem, if not with Anna then with others - DD is just absolutely That Child who is on the receiving end of this sort of stuff.
Her confidence is generally high, though of course I try to help with that when necc too.
I'm not sure what else I can do?
Any cast-iron, sure-fire tips?
Thank you