Quran says:
….whoever kills a soul unless for a soul or for corruption [done] in the land - it is as if he had slain mankind entirely...(Quran 5:32)
So killing of anyone (Muslim or non-Muslim) is a major sin with the exceptions mentioned in above quoted verse.
Islam is a religion of peace and throughout history non-Muslims have lived in peace and harmony under Muslim rule. Muslims ruled India for about a thousand years, and therefore had the power to force each and every non-Muslim of India to convert to Islam or to kill them, but they did not because it is against Islamic teachings. Thus more than 80% of the Indian population remain non-Muslim even after living under Muslim rule. This is a historical fact and clear proof of Islamic teachings of tolerance and respect to other religions.
Below is a very good article which has more evidence from Quran and Sunnah on this subject and hope you will find that helpful to make your case.
https://www.islamreligion.com/articl...islam-part-10/
This website has more than 23 articles on rights of non-Muslims in Islam. And hope your friend will read these with an open mind. If he does then it will be good for him, if he does not then there is no need to argue. Your job is only to convey the message with respect.
https://www.islamreligion.com/catego...d-non-muslims/
Ma'a Salama