On My Way,
This may not apply to every specific nuance of your friend’s beliefs, but likely it will be close. This is from the “Westminster Confession of Faith” Chapter 11 Art. 5 on Justification:
God doth continue to forgive the sins of those that are justified: and, although they can never fall from the state of justification; yet they may, by their sins, fall under God’s fatherly displeasure, and not have the light of His countenance restored unto them, until they humble themselves, confess their sins, beg pardon, and renew their faith and repentance.
Some Scriptures quoted are the Lord’s Prayer “forgive us our debts, as we forgive our debtors” and of course I John 1:7 and 2:1, you know that, about “if we confess our sins, He is faithful and just to forgive us our sins.”
It seems there’s a tension there about being “saved” and having, for a time, unforgiven sins. I never understood that, maybe I didn’t try hard enough to understand… anyways…
Also some Psalms are cited, like 89:31 and 51:7.
Likely your friend hasn’t thought it all through. He thinks what you believe is weird and blasphemous to God’s grace. But that doesn’t mean he isn’t highly thankful for his faith, he probably loves God very much and is very careful to not live a sinful life.
Verses you might keep in mind to consider are
John 10:27-29 (about nobody being able to snatch the sheep out of the Son’s or the Father’s hands)
I Corinthians 1:7-9 "sustain you to the end"
I Timothy 4:18
Man, if you can pro-actively set forth the Catholic view of Scriptures like those that show the confidence that people who live a grace-filled life can have in regards to their salvation, before he gets the chance to “use them on you” so to speak, you will be one step ahead and it will probably give him plenty to mull over. Used along with the verses cited above by other posters, it will show that you are honestly trying to deal with the whole scope of the issue, and not leave anything out.