In this situation, are you allowed to use the horn?

Why?

You must not use the horn to hurry the pedestrian using a walking stick. You should stop and allow them to cross. In a built-up area, you may sound the horn only when necessary because of immediate danger.

art. 29 ust. 1 i ust. 2 pkt 1 i 2 Prawa o ruchu drogowym