In a nutshell, DD had a similarly quiet friend after starting the current school a year ago in yr5. Best friends, playdates, sleepovers etc. A few weeks ago DD and another of the wider friendship group were chatting about this friend 2 coming over one day after school. Since then, 'best' friend has deliberately been costing up to friend 2 and excluding DD. She's back to normal when f2 isn't around.
This morning I dropped DC off, DD went in to share some exciting news, which I saw her do. By the time the bell went, the group of girls walked past me, friend 1and 2 obviously walking separately to DD, giving me a sly side eye as they passed. Previously they would have smiled and said hi, so something is obviously going on. DD has reported no problems or fights, except for feeling excluded when these two get together.
We have a few things going on (illness/dh away) at the moment, so I want DD to be stressed as little as possible by these (not v good) friends, but how do I explain and rationalize their behaviour in a way she will understand without causing greater upset? They are mostly 11, friend 2 is 12.