Something strikes me strange when someone who has left - or openly rejects the Catholic Church - wants to be upheld as a role model for the newly Baptized be being named the Christian Witness; being a Godparent would not be possible as one must be a Catholic who fulfills all canonical requirements. Any guidance or adice? God bless. FT
Technically there is no such things as a “former Catholic.” Once baptized Catholic, or received into the Church, that person is always considered a Catholic. Therefore, as a Catholic he or she cannot serve as a Christian witness, that is reserved for those who are validly baptized outside of the Catholic Church.
As a Catholic who does not practice the faith, they would be ineligible to serve as a God parent since they would obviously not be leading a life of faith in keeping with the function to be taken on (canon 874 §1, 3).