The short answer is no, you don't owe that broker a commission.
If you are unwilling to pay a commission, you should make that clear if you want to avoid any disputes. It is the broker's responsibility to find out, but in reality it would be an easy mistake to make. It's best to simply be forthcoming.
If you would be willing to pay the commission, that rate should be agreed to before any showings. If you are unwilling to pay the commission, you run the risk that the broker simply takes their client and moves on.
It's not very common for rentals, but it's possible the Broker has an agreement with the prospective tenant so that they will cover the commission if the landlord doesn't. If it's his "buddy," the broker may simply be willing to help regardless of whether you pay.
If they actually have someone that's interested in the apartment they will likely ask whether you'd pay a commission. If you are unwilling to do so you run the risk that they won't introduce you to their client.