May a priest be a guest in the congregation at a Catholic wedding?
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.
By Tom Nash