Need clarification for HEXTECH CHEST

So i was playing katarina and my friend Xin. He got the S i'Ve got a chest too..but now i can'T get a chest on katarina even though i never got an S- s s+ on her...highest rank achieved is A+. Is it normal. because i was playing katarina it means i'll never get the free kata chest that is so easy to get ? Does that means that If I play Galio and suck but my firend rock on Katarina and he never got a chest i'll get my Galio chest easy ? is that the way it work ? I dont undertsand the Friend mechanic behind the chests. Some one help me plz :)