8. Mourning after a relative has become a heretic:[1]

Rabbeinu Gershom sat Shiva for 14 days for his son who became a heretic [and died[2]]. This however is not required from the letter of the law, and was done simply to relieve his pain over the fact that he did not merit to repent.[3] [From here one can learn that when a child r”l becomes a heretic, and serves idolatry, that the parents need to cry and mourn [at the time that the child becomes a heretic, even if still alive].[4] Practically, so is the custom, however only in relation to parents. Other relatives however do not need to mourn.[5]]

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[1] Taz 340:3; Beis Yosef 345; Beir Hagoleh 345; Hagahos Ashri end of Moed Katan; Mordechai end of Moed Katan; Hagahos Maimanis 11; Or Zarua 2:428; Maharam Merothenberg Semachos 37; Teshuvos 544; Hagahos Semak 97

[2] So writes Taz ibid; Levush 345; Chasam Sofer 326 and so is implied from Maharam Merothenberg and other Rishonim, thus implying that he only sat 14 days of Shiva after he died, one time for his body and one time for his soul. However, from the Or Zarua ibid this is omitted and the implication is that he sat Shiva as soon as his son became a heretic, as it is even forbidden to sit Shiva for a heretic. So learns Chochmas Adam 156:6; See Makor Chesed on Sefer Chassidim 190

[3] Beir Hagoleh ibid; Beis Yosef ibid

[4] Sefer Chassidim 190; Beis Lechem Yehuda 345

The reason: As if one mourns a person who lost his body, certainly one should mourn a relative who lost his soul. [ibid]

[5] Beis Lechem Yehuda 345; Degul Merivava 345