18.104.22.168 Certain Roles in Catholic and Non-Catholic Ceremonies
1. Catholic ministers should advise Catholics to avoid serving in a role at a non-Catholic ceremony when it would be offensive to divine law or the Catholic faith, express a manifest religious indifferentism, or give scandal.
2. Since the pastor is obliged to prevent liturgical abuse, it is within his duties to judge if a non-Catholic is not suitably disposed or not able to perform a particular role properly. 
3. Doubtful cases can be referred to the chancery or ecumenical officer.
 c. 528 §2.