One is never an ex-Catholic. Once baptized a Catholic the person is ALWAYS a Catholic. They may be away from their faith, but they still have the Catholic mark of Baptism on their soul.
All they have to do to come back to the church is sit down with a priest and get their questions answered....and of course, repent of their sins.
There is nothing in scripture to support that claim....salvation does not come from any single church or denomination proclaiming it is exclusively theirs to offer, regardless if they proclaim as much.
Mormons, Islam and many others also make this claim.....it's a form of "exclusiveness" promoted by those who desire absolute conformity to their church or organization........Christ proclaimed clearly that no man can come to the Father apart from Him....so church affiliation means squat without Christ as ones Savior.
It's Christ himself who offers salvation and it comes thru the work of His spirits work in the heart and the mind of an individual..... Even the Pope acknowledges Christians are in every branch and not exclusively catholic churches.
On the other hand I do believe if one is a Christian thru Christ, he is secure and God will deal with those who leave the fold. The question might rather be were they truly a Christian to begin with or did they just practice "churchianity". For all churches can attest to having "pew sitters" along side those who truly know Christ.