I came across this English sentence when I was reading an article:
Have you ever met anyone on holiday who you were friends with at school or university?
Wasn't it supposed to be "friend" in place of "friends" there?
Be friends with is the ordinary English idiom: it expresses the notion that the friendship is not a relationship from one person to another but exists between two (or more) people.
When you are speaking of just one side of the relationship you may say: John is a friend to Amy (but that's fairly rare these days, and tends to be used specifically of John's actions on her behalf) or John is a friend of Amy's, signifying that he is one of her (many) friends.
Using "were friends with" is correct. The word friend gets pluralized when "friends with" alludes to a friendship between you and the other person:
When you change the wording of the sentence to avoid the phrase "friends with", then you'd use the singular: