One day kids this age are best friends, and the next...you never know.
Some friendships change or fade as preteens switch schools or figure out who they are. But with best friends, if one kid decides to end a friendship that the other still values, it can really hurt.
It's tough to see your child upset, but limit your role to emotional support (unless your child's been bullied; then you should involve other adults). To help:
Don't try to salvage the old friendship. Pursuing a relationship that's over makes it harder for your child to move on, says Fred Frankel, Ph.D., author of Good Friends Are Hard to Find.
Let her feel sad. Listen, and be sympathetic. Talk about how hard it is when people change as they grow up.
Help her make new friends. Find after-school classes related to her hobbies, or ask her about kids she talks to at school. Suggest low-key get-togethers ("Maybe you guys could meet at the parade this weekend").
Talk about being a good friend. If you see your child being, say, bossy or possessive on a playdate, intervene. Later, talk about what your child wants to give and get in her friendships.