My daughter is getting married to a Lutheran man and they plan on getting married in the Lutheran Church. Can a Catholic priest assist the Lutheran pastor at the ceremony?
Canon 1127 §3 of the Code of Canon Law prohibits this type of ceremony:
[T]here is not to be a religious celebration in which the Catholic who is assisting and a non-Catholic minister together, using their own rites, ask for the consent of the parties.
Even so, your daughter might be able to obtain a dispensation from canonical form in order to have the marriage celebrated in a Lutheran ceremony (with no assistance from a Catholic priest). I recommend that she contact her pastor to discuss the matter.