My son is being married outside the Church by an episcopal minister. He was raised Catholic and we have several good friends that are priests. If they are not allowed, is it a canon law issue or a rubric issue? Thank you.
Its generally a matter of propriety. A priest does not simply represent himself, through ordination he also represents the Church. As a Catholic your son is canonically required to be married in the Church. So we are thus faced with two potential situations:
If your son has received a dispensation to be married outside the Church (dispensation from canonical form) then the priests most certainly could attend the wedding, but could not concelebrate in any way since the Catholic Church does not recognize the validity of the Episcopal Eucharist.
If your son has not received a dispensation from canonical form then his marriage would be invalid. It would be highly inappropriate for a Catholic priest to attend and give the appearance of approval to a Catholic entering into an invalid marriage. Surely one of them could help him obtain the dispensation.