Is it haram to cook non-halal meat and serve it to a Christian? Bearing in mind that you don’t eat it, but just cooking it and serve it.
First of all, selling haram things is definitely forbidden, so you cannot sell any food that is haram:
I saw the Messenger of Allah (ﷺ) sitting neat the Black stone (or at a corner of the Ka'bah). He said: He (the Prophet) raised his eyes towards the heaven, and laughed, and he said: May Allah curse the Jews! He said this three times. Allah declared unlawful for them the fats (of the animals which died a natural death); they sold them and they enjoyed the price they received for them. When Allah declares eating of thing forbidden for the people, He declares it price also forbidden for them. The version of Khalid b. 'Abd Allah al-Tahhan does not have the words "I saw". It has: "May Allah destroy the Jews!" (Abu Dawud)
Basically, if you can't eat it or use it you can't make money off of it.
In the case of giving the food for free, the laws of Allah apply to everyone regardless of whether they acknowledge it or not. So as muslims, we cannot help someone do a sin (eating haram is a sin). In fact we are supposed to do the opposite; we are supposed stop it from happening:
“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al‑Munkar (polytheism, disbelief and all that Islam has forbidden), and you believe in Allaah” (3:110)
Since we are supposed to forbid sin, obviously it is forbidden to actively help it happen (by giving someone haram food).