The Rebbe's guidance on intermarriage:
A grave matter with lasting consequences. The Rebbe warned strongly and urgently against intermarriage or living with a non-Jewish partner, calling it a personal tragedy and a threat to the Jewish people, whose harm lasts far longer than an ordinary mistake, for both partners and for the generations after (IK 1322, 10502, 10945).
For parents whose child is involved with someone not Jewish: parents bear real responsibility and must act, through both action and prayer (IK 8745). Act promptly but discreetly: involve trusted people in the community, quietly create opportunities for the child to meet suitable Jewish partners, keep the home's spiritual safeguards in place, and avoid direct confrontation that could push the child away (IK 1771, 6197). Where a conversion is claimed, look into it discreetly (IK 6197).
See also: שידוכים – צד שאינו שומר מצוות.