He is one of the NPC's you can start the Ardougne Tasks by talking to, and will give you the Ardougne cloak 1 as a reward for completing the easy tasks. If you lose the cloak, you can talk to him for another.
If spoken to while wearing full Mourner Clothes, he will ask for his payment and requests that the Head Mourner be told about it, as he won't be keeping the charade any longer for free. This hints towards his collaboration with the Mourners, and is further cemented after the events of Plague's End, where he will advise the player under the disguise of full mourner clothes to escape the reprisals from the citizens of West Ardougne while lamenting how the charade cannot be kept.
If the player isn't wearing the mourner disguise and speaks to him after Plague's End, he simply questions his ability as a doctor and claims ignorance to the dyed sheep.