Liturgical Norms

Is it okay for a lay person to preach at Sunday Mass? 

Answer

This is clearly a liturgical abuse. The General Instruction of the Roman Missal states:

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)