The word “knei” also means to acquire, to make a kinyan. When a man marries a woman he makes a kinyan, he is acquiring a wife. She is not being sold to him (though I bet a wife or two has felt that way in the past…). So one should regard a good friend as he does an important and crucial acquisition in life.
I had another thought as I was typing. Hopefully I have the spelling correct (if not I am sure someone will immediately inform me of same…). The word “kaneh” in Hebrew, means something upon which we lean, i.e. a cane (cognate if I ever heard one). Derech Agav, I believe Egypt is referred to in Navi as a “kaneh ratzutz (meaning an undependable ally, though appearing at first to be reliable).”
A true friend is one upon whom we can lean, in times of tzoros. As I said, if its root is spelled the same way as “knei” with a koof and not a chof, then perhaps I have a little pshat of my own here.