Boy: "I'm going to the movies..."
Mom: "With friends?"
Boy: "I'm going with Jane."
Mom: "I don't like that Jane. She's not a good girl for you. I don't know what you see in her. Why can't you take Katie from down the street, or that Russian girl you met online."
Boy: "Mom, stop! That Russian girl lives IN RUSSIA!"
Mom: "Well, I hear Jane sleeps around a lot..."
Boy: "I know, mom! That's why I'm taking her out!!!"
No, "With friends?" totally stand on its own in English.
Don't know about that (I doubt they do), but this is plural, not singular. "With a friend - Mit einem Freund". The -en suffix comes from this: "laerteginez: Dear friend, there's a rule in german: Nouns in plural that do not already end in -n or -s gain an -n in the dative case."