You friend is peddling the Khazar theory of origin for Ashkenazi Jews. this theory holds that most Ashkenazi Jews are not descended from Rhineland Jews expelled during the Crusades, but from the Khazars, a Oghur Turkic people, some of whose leadership did convert. Although linguistics and simple history precludes this, this was a theory peddled by some Hugarian Jews (they wanted to be seen as inheritors of the Khabars, as Khazar group that joined the Magyars, thus making them authentic Hungarians) as well as communists, the Judeophobic right, and Islamist and Arab Nationalists. However, genetic testing has shown the opposite. Not only are Ashkenazi Jews related primarily to other Jews, but we are exceeding endogomous. Most Eastern European Jews are descended from just 4 women and are genetically very similar to Sephardic Jews. This is not to say that there was no intermarriage. In fact, there was quite a bit, especially with Sorbs and Ruthenians.
Personally, I am descended from Khazars, in part, if family lore can be believed. My anscestors fought the Caliphate from 640 until 737, keeping Islam out of Europe in 3 bloody wars that mad Tours look like a little raid.
You know, Khazar is almost always heard in a negative context, so I was surprised to hear several different Turks (including Turkmen) use it in a positive way on separate occasions. Basically it went, “We love Jews because they are Khazars.” & “Khazars are Turkic people, so Jews are our brothers.” Never heard it used as a compliment by anyone else.