-Maybe Rachel thinks the definition of friendship means helping Castiel with the war more - but again, not Sam and Dean's fault...they have offered to help, and Castiel has refused. He's been keeping them at bay and not telling them very much, so what else are they supposed to do? They are just two humans and this is an angelic war.

Well, during this time Castiel was also off conspiring with Crowley. It is never stated, so this is my fanwank, but telling his angel followers that he is leaving because Sam and Dean called for help AGAIN would make a nice excuse to cover any meetings with Crowley or any undercover operations he was doing with Balthazar. It's just a handwave, but it would explain why Rachel is so pissed, she thinks Sam and Dean are calling for Cas far more than they actually did.