When she is in trouble but doesn't ask for your help, you might want to say "She and I have been friends since elementary school." She is important and you're ready to help her, so you wonder why she doesn't come to you.
@Ladybug9 Yes, you use this when talking to someone. For example, Jenny is your friend who is in trouble. You're talking to Amanda, a mutual friend, about Jenny's problem, and say " since Jenny and I have been friends since elementary school, I'd be happy to to help Jenny."