When the gases sulphur dioxide and hydrogen sulphide mix in the presence of water the reaction is :

SO₂+2H₂S=2H₂O+3S

Here hydrogen sulphide is acting as :

A. Oxidizing agent

B. Reducing agent

See it is very simple. first you need to see which substance has hydrogen before reaction and which one has later. thus H2S has hydrogen. Now see which product has hydrogen, i.e. H2O. now see where does O belong from. O belongs from SO2. thus SO2 is reduced to H2O and H2S is oxidised to H2O. always remember that oxidisation is reverse of reduction.

ie H2S is act as Reducing agent.