I remember seeing an article earlier on FR that if you were once baptised as a Catholic, it holds for life, no matter what. There were many comments agreeing with it, as well.
Baptism is a sacrament cannot be “undone”.
It’s not just a catholic thing - a person cannot be “unbaptized” yes.
The catholic church recognized many baptisms outside of the church.
That Mel was baptized catholic prior to vatican II - does not make him a catholic now, just like any catholic who leaves the church for whatever reason.