If your friend wants a mellow dog who gets along with everybody, then perhaps she would like to get an adult dog from a rescue. They can tell her what the dog's personality is like. It's more of an individual thing than a breed thing. There are high-strung and mellow individuals of each breed.

I've met lots of mellow shelties. I have one. He gets along with everybody and rarely barks (unlike the stereotypical Sheltie, who barks incessantly). That's what I mean about individuals of the same breed being different.