All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for you to write a report describing what had happened as a complaint because of being wronged as Allaah says (what means): {Allaah does not like the public mention of evil except by one who has been wronged. And ever is Allaah Hearing and Knowing.} [Quran 4:148] Also, as a way of advising the general public so that the owners of that restaurant would be obliged to check their food and take safety measures so that they would not cause the death of people with their rotten and contaminated food.
The Prophet said: "Religion is advice". They said, "For whom?'' He replied, "For Allaah, His Book, His Messenger and for the leaders of the Muslims and their masses." [Muslim]
Allaah Knows best.