Can a Catholic participate in the wedding of a Catholic and a non-Catholic Christian in a Protestant church (e.g., wedding party, best man, usher, readings, singing solo, saying grace at reception, etc)?
If the Catholic partner has the necessary dispensations from his bishop to marry a non-Catholic Christian in a non-Catholic ritual, there is no problem with a Catholic friend or relative participating in a non-Catholic wedding. Such participation should reflect the guidelines for Catholics participating in Protestant services set out in the Directory for the Application of Principles and Norms on Ecumenism.
If the Catholic partner does not have the necessary dispensations, it may be possible, with just cause, for the Catholic friend or relative to attend the wedding, but I cannot recommend participating in such a wedding as a member of the wedding party or as a participant in the service.
Should I Attend? by Catholics United for the Faith