Assalamualaikum. What is the ruling of eating food cooked or prepared by non-Muslims even if it is from halal sources?
Alhamdulillah, praise and thanks to Allah for the countless blessings He has blessed us all with. Blessings and salutations to the Prophet Muhammad PBUH, his wives, his family, companions and all those that follow his teachings to the day of judgement.
It is not wrong for a Muslim to eat food cooked or prepared by non-Muslims made from milk, vegetables and all other foods except for their slaughter.
Qatadah Rahimahullah said:
لَا بَأْسَ بِأَكْلِ طَعَامٍ الْمَجُوسِ مَا خَلَا ذَبِيحَتَهُ
It is not wrong to eat food of the Zoroastrians except for their slaughter
Narrated by Abd al-Razzaq in his Musannaf (10155)
The same for what is mentioned by Imam al-Qurtubi Rahimahullah in his tafsir:
وَقَالَ أَبُو عُمَرَ: وَلَا بَأْسَ بِأَكْلِ طَعَامِ عَبَدَةِ الْأَوْثَانِ وَالْمَجُوسِ وَسَائِرِ مَنْ لَا كِتَابَ لَهُ مِنَ الْكُفَّارِ مَا لَمْ يَكُنْ مِنْ ذَبَائِحِهِمْ
“Abu Umar said: and it is not wrong to eat the food of the idolaters, Zoroastrians and all of no book (non-people of the book) among the non-Muslims except for their slaughter” 
It is permissible for a Muslim to eat food cooked or prepared by non-Muslims (non-people of the book) like the Hindus, Sikhs, Buddhists and others as long as the food is halal and not from their slaughter.
So, there is no problem for a Muslim if he is invited to the house of a non-Muslim friend then he is served by food to be eaten as long as it is halalan toyyiba.
However, this is not a denial towards the priority of being wara’ in our eatery. It is prioritized (afdal) for our food to be cooked by a pious people and prepared with taqwa. In a hadith, the Prophet PBUH once said to remind his ummah:
لاَ تُصَاحِبْ إِلاَّ مُؤْمِنًا وَلاَ يَأْكُلْ طَعَامَكَ إِلاَّ تَقِيٌّ
Associate only with a believer, and let only a God-fearing man eat your meals.
Sunan Abu Dawud (4832)
 See Al-Jami’ li Ahkam al-Quran. Dar al-Kutub al-Misriyyah, (6/77)