As a final point to the OP, once you have informed them of the Talmud or Oral Torah you should quote some to them. Here are some choice passages.
Baba Mezi’a 114b: “…it has been taught: R. Simeon b. Yohai said: The graves of Gentiles do not defile, for it is written, And ye my flock, the flock of my pastures, are men; only ye are designated 'men'.”
‘Abodah Zarah 36b-37a: “From what age does a heathen child cause defilement by seminal emission? — He replied to me: From a day old; but when I came to R. Hiyya, he told me: From the age of nine years and one day. When I then came and discussed the matter with Rabbi, he said to me: Abandon my reply and adopt that of R. Hiyya who declared: From what age does a heathen child cause defilement by seminal emission? From the age of nine years and one day, for inasmuch as he is then capable of the sexual act he likewise defiles by emission. Rabina said: It is therefore to be concluded that a heathen girl [communicates defilement] from the age of three years and one day, for inasmuch as she is then capable of the sexual act she likewise defiles by a flux. This is obvious! — You might argue that he is at an age when he knows to persuade [a female] but she is not at an age when she knows to persuade [a male, and consequently although she is technically capable of the sexual act, she does not cause defilement until she is nine years and one day old]. Hence he informs us [that she communicates defilement at the earlier age].”
Kethuboth 11b: “When a grown-up man has intercourse with a little girl it is nothing, for when the girl is less than this (three years old), it is as if one puts the finger into the eye;”
Babi Mezi’a 24a: “Come and hear: If one finds therein a lost object, then if the majority are Israelites it has to be announced, but if the majority are heathens it has not to be announced”
Baba Kamma 113a: “We suppose the customs-collector here to be a heathen publican as it was taught: 'Where a suit arises between an Israelite and a heathen, if you can justify the former according to the laws of Israel, justify him and say: 'This is our law'; so also if you can justify him by the laws of the heathens justify him and say [to the other party:] 'This is your law'; but if this can not be done, we use subterfuges to circumvent him.”
Sanhedrin 59a: “A heathen who studies the Torah deserves death, for it is written, Moses commanded us a law for an inheritance; it is our inheritance, not theirs. Then why is this not included in the Noachian laws? — On the reading morasha [an inheritance] he steals it; on the reading me'orasah [betrothed], he is guilty as one who violates a betrothed maiden, who is stoned.”