The General Instruction of the Roman Missal (GIRM) is clear that the homily may never be entrusted to a lay person:
“The Homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person. 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.” (GIRM 66)
The recent document *Redemptionis Sacramentum * (RS) clarifies this further:
“It should be borne in mind that any previous norm that may have admitted non-ordained faithful to give the homily during the eucharistic celebration is to be considered abrogated by the norm of canon 767 §1. This practice is reprobated, so that it cannot be permitted to attain the force of custom.
“The prohibition of the admission of laypersons to preach within the Mass applies also to seminarians, students of theological disciplines, and those who have assumed the function of those known as “pastoral assistants”; nor is there to be any exception for any other kind of layperson, or group, or community, or association.” (RS 65-66)