Can you buy fries from a non-Muslim, who sell Haram things like sausages? However, I asked them if the fries touched these and they said no. Can we trust them and buy fries from them, as there isn't a Muslim chip shop near us?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Trading and dealing with non-Muslims is permissible except in those field that might be used against Muslims by the non-Muslims who are in the state of war with Muslims.

Imam Al-Bukhari entitled a chapter in his book by this heading: “Buying and selling with polytheists and the enemy.” He mentioned under this chapter the Hadeeth of ‘Abdur-Rahmaan ibn Abu Bakr who said: “We were with the Prophet when a tall polytheist with long matted, unkept hair came driving his sheep. The Prophet asked him, “Are those sheep for sale or for gifts?” He replied, “They are for sale.” The Prophet bought one sheep from him.”

If we are not certain that something has been mixed with what is forbidden, then purchasing it is permissible and we do not bother to investigate about it.

Allah knows best.