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

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)

Redemptionis Sacramentum reiterates GIRM 66 and adds the following:

• 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 [law]. This practice is reprobated, so that it cannot be permitted to attain the force of custom. (RS 65)
• If the need arises for the gathered faithful to be given instruction or testimony by a layperson in a Church concerning the Christian life, it is altogether preferable that this be done outside Mass. Nevertheless, for serious reasons it is permissible that this type of instruction or testimony be given after the priest has proclaimed the Prayer after Communion. This should not become a regular practice, however. Furthermore, these instructions and testimony should not be of such a nature that they could be confused with the homily, nor is it permissible to dispense with the homily on their account. (RS 74)
• As was already noted above, the homily on account of its importance and its nature is reserved to the priest or deacon during Mass. As regards other forms of preaching, if necessity demands it in particular circumstances, or if usefulness suggests it in special cases, lay members of Christ’s faithful may be allowed to preach in a church or in an oratory outside Mass in accordance with the norm of law. This may be done only on account of a scarcity of sacred ministers in certain places, in order to meet the need, and it may not be transformed from an exceptional measure into an ordinary practice, nor may it be understood as an authentic form of the advancement of the laity. All must remember besides that the faculty for giving such permission belongs to the local ordinary [bishop], and this as regards individual instances; this permission is not the competence of anyone else, even if they are priests or deacons. (RS 161)

The document goes on to explain that abuses such as this “are not to be considered of little account” and are to be “carefully avoided and corrected.” (RS 174)

Any Catholic, whether priest or deacon or lay member of Christ’s faithful, has the right to lodge a complaint regarding a liturgical abuse to the diocesan bishop or the competent ordinary equivalent to him in law, or to the Apostolic See on account of the primacy of the Roman pontiff. It is fitting, however, insofar as possible, that the report or complaint be submitted first to the diocesan bishop. This is naturally to be done in truth and charity. (RS 184)