Is it okay for a Catholic to participate in a United Methodist confirmation of a youth for whom I am one of the sponsors? I do not know what my part may be in the ceremony but was told secondhand that I may need to participate in some way.
Since confirmation is not a valid sacrament in Protestant churches, Catholics cannot act as sponsors for Protestant confirmations. Since you have apparently already accepted the invitation to be a sponsor, I recommend gently explaining to the young person and his or her parents that you have come across additional information about the ability of Catholics to participate as Protestant confirmation sponsors and regret that you will not now be able to be this young person’s sponsor.
As for attending the ceremony as an observer, that is a trickier question. Ordinarily, it is not recommended that Catholics attend invalid sacraments because to do so can cause non-Catholics and other Catholics to think that you, as a Catholic, are recognizing the validity of the sacrament by your presence at its celebration and thus can be the cause of scandal. The Catechism of the Catholic Church says about scandal:
Scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense (CCC 2284).
Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized. It prompted our Lord to utter this curse: “Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea.” Scandal is grave when given by those who by nature or office are obliged to teach and educate others. Jesus reproaches the scribes and Pharisees on this account: he likens them to wolves in sheep’s clothing (CCC 2285).
Sometimes, however, there may be proportionate reasons to accept the risk of the possibility of scandal because those proportionate reasons are sufficiently serious enough to outweigh the accompanying risk.
In the case we’re considering, it seems that you accepted the offer to be a sponsor to this Protestant confirmand before you knew that Catholics cannot act as confirmation sponsors for Protestant confirmands. You must now bow out of accepting that position, and to do so will be difficult for you to do and for the confirmand and his or her parents to understand. In and of itself, having to renege on a promised duty can be shocking to people’s sensibilities, but you have no other choice about that. It is possible that you can lessen the sense of shock by attending the event as a non-participating observer. And the scandal that might otherwise be caused by your attendance at an invalid sacrament might be mitigated by your having backed out of being a sponsor.
You will have to consider your options carefully and decide what the ramifications will be if you attend this confirmation. When it comes to attendance at invalid sacraments, the Church does not explicitly forbid such attendance to Catholics, but enjoins upon Catholics the responsibility to do all that they can to witness to the truth.