From Longman Dictionary of Contemporary Englishbe friends (with somebody)be friends (with somebody)to be someone’s friend I’ve been friends with the Murkets for twenty years. → friend
Examples from the Corpus
be friends (with somebody)• Ministers are at ease with the former, they are friends exploring a situation.• Then they starved together, and now they were friends.• Louisa had lied when she said they were friends.• They were friends of his, real estate developers with no expertise whatsoever in media, much less multimedia.• He asked me to be friends again, but I couldn't forgive him.• We were friends, on Sunday afternoons we went running in the lower meadows.• He gave the Bookman a big hug and hoped they would be friends for a long, long time. 