A Priest in the Pew Is OK to Do

While priests typically serve as celebrants or concelebrants at Catholic weddings—especially when the wedding takes place within the sacrifice of the Mass—there is nothing in Church law that would preclude their being in the congregation. I would double-check with the priest in question. If he wants to concelebrate, I would let him do so, and hopefully there’s no overriding objection by the main priest celebrant of the wedding or the bride or groom. Given that he’s already a wedding guest, I would infer that that’s not an issue.

If the priest doesn’t want to concelebrate, I would respect his decision, and he can sit with the congregation in his normal, non-liturgical clerical garb. In such case, I would also let the main priest celebrant of the wedding know, so that there is no confusion if he sees Father the day of the wedding but not coming into the sacristy to vest for the celebration.

The same would apply to non-wedding Masses. Though unusual, sometimes a priest for various reasons may be a participant in the congregation, including because his congregation directs him to only offer the Mass according to the Extraordinary Form, i.e., according to the older Tridentine/Latin Mass rite.