Hi, "John explained the theory to his taciturn friend at Steven's Cafe who was listening to him with bemused interest."
1. Is the above construction correct?, Yes. and 2. What does "who" qualify: "taciturn friend" or "Steven's Cafe"? 'Friend'. ''Stephen's Cafe' is not a person.
You need a comma after Cafe, to make the 'who' clause non-identifying information.