"Unbelievers are those who say: 'God is one of three.' There is but one God. If they do not desist from so saying, those of them that disbelieve shall be sternly punished." (Surah 5:73)
"The God will say: 'Jesus, son of Mary, did you ever say to mankind 'Worship me and my mother as gods besides God?' 'Glory to You, 'he will answer, 'how could I ever say that to which I have no right?" (Surah 5:116)
Ask them why their all-knowing God would say, "Unbelievers are those who say 'God is one of three'.
Acknowledge that the Trinity is an admittedly confusing concept, but that at its core, it's pretty simple: God is one, but God sometimes takes different forms. So press them on why their God showed a fundamental lack of understanding of basic Christian theology.
And then move to 5:116. Ask them why God would ask Jesus such a question. Ask them if they have in fact read the Gospels of Matthew, Mark, Luke and John. If they say they have, ask them where in the Gospels Jesus told his followers to make an idol of his mother.
Of course, they will have no response, so press them on it: "Obviously, the Bible makes no mention of holding the mother of Jesus up as an idol, so on the last day, why will God ask him if he did? The question is totally irrelevant. God might as well ask Jesus if he told his followers to devote at least ten hours a week to the art of breakdancing.