In this answer I provided a sample sentence:
You are friend with Toby.
and as a result I received a comment from @JasonBassford:
*Both of your example sentences are ungrammatical. You can't be friend with someone. You can be a friend of someone or you can be friendly with someone.*
Now, I tend to trust JasonBassford (as a result of seeing hos posts here on ELL), but a dilemma remains.
From my point of view, friendship and friendliness do not convey the same meaning.
I can be "friendly" with a dog or with some foreigner on the street, but that will not make us friends. Friendship (from my point of view) is much deeper and much more complex than friendliness.
So, to be explicit: is there another way to express friendship? I feel that something is missing, but I am not sure what that is.
As one alternative, I think about "to be friends with" (friend + s = plural):
You are friends with Toby.
but I cannot decide if it is fine / idiomatic, or if there is something else missing.