Malbim: It is also a Yisraelis, if she is not his wife. Nochriyah is specifically from another nation. Normally, one loves a Nochriyah only if she entices him with Chelkas Leshonah (6:24), therefore it says "Amareha Hechelikah."

Malbim (according to Melitzah): They are investigations foreign to Chachmas ha'Torah. She can be his wife if he will take her with Chupah and Kidushin. One can use these Chachmos for matters of Torah. Nochriyah is heresy and idolatry, which are totally foreign to Torah, just these make their sayings smooth with philosophical comparisons that draw to heresy until he is drawn after them. Chachmah saves from both of them.