Q. Is one permitted to leave Eretz Yisroel to visit an ailing parent who has intermarried?

A. See questions 646 and 647 in regards to sitting shiva, mourning and burial for someone who intermarried, and is regarded for some issues as having abandoned Judaism.
However, Horav Shlomo Miller’s Shlit’a opinion is that if there is a chance that he can influence his parent to become a proper baal teshuva, or even begin to think about doing teshuva, he may visit.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a