Recently, viral on social media that non-Muslims who are Sikh giving vegetarian food cooked and prepared by them to flood victims. Thus, what is the ruling for the food, although the food is from a halal source? Hope for an explanation.
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
We begin with the Allah SWT’s statement:
يَا أَيُّهَا النَّاسُ كُلُوا مِمَّا فِي الْأَرْضِ حَلَالًا طَيِّبًا
“O mankind, eat from whatever is on earth [that is] lawful and good,” 
The above verse shows that the food that we eat must fulfil the two criteria which are halal and good.
Syeikh al-Sa’di when commenting on this verse said that the verse is for all mankind, regardless of whether they are believers or not, to eat whatever that is on this earth from grains, fruits and animals that is halal for them to take (which means food that is eaten and benefitted from), not taken through force, stealing, prohibited muamalat (trade) or other prohibited matters. While the word ‘tayyiban’ means not from something bad (dirty and disgusting) such as carcass, blood, pork and others. Thus, the verse is evidence that the original ruling for anything (to be eaten and benefitted from) is permissible. 
Choosing a halal and good food is one of the factors our supplication is granted by Allah SWT. The Prophet Muhammad PBUH once said:
الرَّجُلَ يُطِيلُ السَّفَرَ أَشْعَثَ أَغْبَرَ، يَمُدُّ يَدَيْهِ إِلَى السَّمَاءِ، يَا رَبِّ، يَا رَبِّ، وَمَطْعَمُهُ حَرَامٌ، وَمَشْرَبُهُ حَرَامٌ، وَمَلْبَسُهُ حَرَامٌ، وَغُذِيَ بِالْحَرَامِ، فَأَنَّى يُسْتَجَابُ لِذَلِكَ؟
“A person who travels widely, his hair dishevelled and covered with dust. He lifts his hand towards the sky (and thus makes the supplication): “O Lord, O Lord,” whereas his diet is unlawful, his drink is unlawful, and his clothes are unlawful and his nourishment is unlawful. How can then his supplication be accepted?” 
Food Cooked by Non-Muslims Who Aren’t from the People of the Book
Regarding this matter, it is not an offence for a Muslim to eat food cooked or prepared by non-Muslims who aren’t from the People of the Book from milk, vegetables, fruits and all types of food except the animals they slaughtered. Furthermore, there is a narration from Qatadah who said:
لَا بَأْسَ بِأَكْلِ طَعَامِ الْمَجُوسِ مَا خَلَا ذَبِيحَتَهُ
“It is permissible to eat the food of the Majus (Magian) except their slaughter.” 
Moreover, al-Qurtubi also cited Abu Umar who said that it is permissible to eat the food of idolators, Majus and other non-Muslims who don’t have a book (which means not from the People of the Book) except the animals they slaughter. Likewise, for food that doesn’t need for slaughtering according to syarak except for cheese because it originates from rennet (carcass) – if the animal is not slaughtered according to syarak. 
Furthermore, the halal status of food is not considered by the person cooking or preparing it. However, the guide is based on the ingredients and utensils used to prepare the food. In other words, it must be made from halal and clean ingredients as well as the utensils used to prepare the food is not used to cook prohibited food.
Hence, it is not an issue for a Muslim to eat the food cooked or prepared by non-Muslims (not from the People of the Book) such the Hindus, Sikhs, Buddhists and others, as long as the ingredients used are halal and its purity is certain and not from their slaughter.
Especially, in flood and post-flood situations where flood victims and volunteers need food. Thus, it is permissible to take the food from non-Muslims especially if it is cooked for vegetarians.
May Allah SWT give us a clear understanding in religion and protect us from prohibited matters. Ameen.