18.104.22.168 Catholic and Non-Catholic Ministers at the Same Celebration of Marriage
1. A priest or deacon who assists at the Catholic celebration of mixed marriage, where it is not in the context of a Eucharistic celebration, may invite the minister of the non-Catholic party to read from the Scriptures (except for the gospel), give a brief exhortation or bless the couple when they request this. 
a. This requires that all necessary dispensations or permissions have been obtained, and that only the Catholic priest or deacon receive the consent of both parties and does so according to the Catholic Rite of Marriage.
b. While a baptized Christian may be permitted to serve as a reader or proclaim the general intercessions at a Catholic Eucharist, only a Catholic priest or deacon may preach at the time reserved for the homily at a Catholic Eucharistic liturgy or proclaim the gospel.  (See #4.1.6 and #4.2.2.)
2. A Catholic priest or deacon is permitted to participate in a non-Catholic celebration of a mixed marriage when invited by the non-Catholic celebrant by reading from the Scriptures, giving a brief exhortation or blessing the couple. 
a. This requires that all necessary dispensations or permissions have been obtained, including dispensation from canonical form, and that only the non-Catholic minister receive the consent of both parties according to the discipline of the non-Catholic Church or ecclesial community.
b. However, Catholic priests, deacons and laity are not to preach at Eucharistic or similar celebrations of non-Catholic Churches and ecclesial communities. (See #4.1.6.)
 Ecumenical Directory, n. 158.
 Ecumenical Directory, n. 135 treats reading at non Eucharistic Catholic celebrations by applying 118, which permits the reading of a lesson but does not mention proclamation of the gospel. (See 4.1.6 and 4.2.2.)
 Ecumenical Directory, n. 157.