Well, since you asked, here's what's wrong with assisting at the invalid marriages of AWOL priests
Reardon apparently admits the deed, but adds "If someone can show me the church law I am supposed to have broken, I would be interested, but I'm not aware of it." Does the archdiocesan vicar general really not know what church laws would have been broken by such conduct?
Well, okay, here's two for starters.
Catholic clergy are impeded from marriage (1983 CIC 1087, olim 1917 CIC 1072) meaning that an AWOL priest who attempts marriage does so invalidly and illicitly. Reardon directly facilitated a brother priest's violation of this sacramental canon.
A cleric who attempts marriage, even a civil marriage, incurs an automatic penalty. 1983 CIC 1394.1 (suspension), olim 1917 CIC 2388.1 (excommunication). Reardon directly facilitated a brother priest's violation of this penal canon.
Other church laws (e.g., those leveling penalties against accomplices in ecclesiastical crimes or those regarding the good conduct of clerics in general) could be cited, but these two should be enough to answer Reardon's request for illumination.
Now, here's what I don't understand: How can the vicar general of an archdiocese, even after thirty years of ministerial and administrative experience, still not be able to figure out what is so wrong about a priest's assisting at the civil wedding of an AWOL brother priest?
Update, 16 October 2008: Reardon is not being named bishop of Menevia, instead an auxiliary is being named to take over the diocese.