If a jew marries a gentile (G-d forbid), are the parents required to seat Shiva, tear kria and say Kaddish? If so for how long?

There is a custom- not a halachic obligation- for parents (only parents and not other relatives) to mourn up to two weeks for a son or daughter who has converted to a foreign dogma . The parents tear kriya, sit on the floor as mourners, bemoan the fact that they gave birth to such a child- but do not observe any of the other laws of mourning (Shu"t Tzitz Eliezer 5 Ramat Rachel 42 quoting earlier sources). Today, most Jews who intermarry do not accept Christianity or some other Gentile belief. Also in the Western world most marriages end in divorce so it is likely that the marriage with a non-Jew is temporary. An accepted psak is that it is important for the family of someone who intermarries to keep up the contact with him and try to maintain any links he still has with Judaism and Torah (see Shu"t Tzitz Eliezer 10, 41).