My mother once left a Mass because a nun was delivering the homily. She believes it was heretical—was she correct?
Heresy is “the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same” (CCC 2089). Nonetheless your mother is correct that nuns and other laypeople should not be giving the homily at Mass. Redemptionis Sacramentum states:
The homily, which is given in the course of the celebration of holy Mass and is a part of the liturgy itself, “should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to a deacon, but never to a layperson. In particular cases and for a just cause, the homily may even be given by a bishop or a priest who is present at the celebration but cannot concelebrate.” (RS 64)
Assuming your mother met her Sunday obligation by attending another Mass that day, and assuming that she left quietly and without undue disruption, it was not wrong for her to register her displeasure at a significant liturgical abuse by leaving Mass.