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.
John and I were friends.
I was friends with John.
John was friends with Tom, Amy and Margie.
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:
At Dartmouth, I was best friends with Sharon.
At the reunion, I met a guy I had been friends with back in the 1980s.
Why aren't they getting along? Bill used to be good friends with Ted.
When you change the wording of the sentence to avoid the phrase "friends with", then you'd use the singular:
Have you ever met anyone on holiday who had been your friend at school or university?
Why aren't they getting along? Bill used to be Ted's good friend.