We learned that Cornelia and Bonnie are best friend since the sixth grade. Bonnie seem to always be the leader of the friendship and Bonnie has been helping Cornelia with her shyness since they became friends. We also view Cornelia self realisation that of overcoming her shyness.
Yes, there is a way for the writer to provide the reader with this same information, without providing a flashback. A description of Cornelia would be fine without providing a flashback of how Cornelia and her best friend met. Simply