Our Church allows our Seminarian ( now a deacon) to preach the homily. Our priest sits on the chair watching him. I have been told it is ok because our Bishop allows this. He will preach once a month. Give me concrete evidence if this is allowed or not.
Yes, deacons may preach the homily. The GIRM provides:
66. 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.
Edited to add: I’ll leave it as an exercise for the reader whether this implies that a deacon who, for whatever reason, is not serving liturgically at a Mass can come up to do the homily.
There is your proof.
Once he is a deacon, he is no longer a seminarian, since he has already received the first step in the major orders (diaconate, priesthood, episcopacy). He certainly may preach. Here is the section from Redemptionis Sacramentum:
[64.] The homily, which is given in the course of the celebration of Holy Mass and is a part of the Liturgy itself,142 “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.143 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”.144
[65.] 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 §1.145 This practice is reprobated, so that it cannot be permitted to attain the force of custom.
[66.] 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.146
Since he is now a deacon, the prohibition against seminarians preaching does not apply to him as he is an ordained minister. Now, if it were a regular seminariran, then there would be a huge problem.