Sometimes the term “homily” seems to include talks by lay people. For example: “§ 4. Homilies in non-eucharistic liturgies may be preached by the non-ordained faithful only when expressly permitted by law and when its prescriptions for doing so are observed.” (Ecclesia de Mysterio, Article 3, at
vatican.va/roman_curia/pontifical_councils/laity/documents/rc_con_interdic_doc_15081997_en.html ).
The 2004 Instruction Redemptionis Sacramentum included: “[165.] It is necessary to avoid any sort of confusion between this type of gathering and the celebration of the Eucharist.[271] The diocesan Bishops, therefore, should prudently discern whether Holy Communion ought to be distributed in these gatherings. The matter would appropriately be determined in view of a more ample coordination in the Bishops’ Conference, to be put into effect after the recognitio of the acts by the Apostolic See through the Congregation for Divine Worship and the Discipline of the Sacraments. It will be preferable, moreover, when both a Priest and a Deacon are absent, that the various parts be distributed among several faithful rather than having a single lay member of the faithful direct the whole celebration alone. Nor is it ever appropriate to refer to any member of the lay faithful as “presiding” over the celebration.
[166.] ***Likewise, especially if Holy Communion is distributed during such celebrations, the diocesan Bishop, to whose exclusive competence this matter pertains, must not easily grant permission for such celebrations to be held on weekdays, especially in places where it was possible or would be possible to have the celebration of Mass on the preceding or the following Sunday. ***Priests are therefore earnestly requested to celebrate Mass daily for the people in one of the churches entrusted to their care.”
So there is now considerable sensitivity about Communion Services. Without clear guidelines, I think it is doubtful that it is appropriate to “fill in” because the priest has not arrived as expected.