I have a friend who uses Ephesians 2:8 as evidence for why they are already saved.
I would probably tell them you totally agree with what it says in this verse. However, as a Catholic you were taught to read the Bible as a whole and not just pick and choose the verses you like.
Tell them…
The Church teaches in agreement with Scripture that Christians:
"have been saved" through faith and baptism:
Mark 16:16
16 He who believes and is baptized will be saved; but he who does not believe will be condemned.
Ephesians 2:8-9
8 For by grace you have been saved through faith; and this is not your own doing, it is the gift of God— 9 not because of works, lest any man should boast.
Romans 6:3-4
3 Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? 4 We were buried therefore with him by baptism into death, so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.
they’re “being saved” through cooperation with grace:
1 Corinthians 1:18
Christ the Power and Wisdom of God
18 For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God.
1 Corinthians 15:1-3
The Resurrection of Christ
15 Now I would remind you, brethren, in what terms I preached to you the gospel, which you received, in which you stand, 2 by which you are saved, if you hold it fast—unless you believed in vain.
3 For I delivered to you as of first importance what I also received, that Christ died for our sins in accordance with the scriptures,
2 Corinthians 6:1
6 Working together with him, then, we entreat you not to accept the grace of God in vain.
and that they "shall be saved" if they persevere in the Faith:
Matthew 10:22
22 and you will be hated by all for my name’s sake. But he who endures to the end will be saved.
Romans 2:6-7
6 For he will render to every man according to his works: 7 to those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life;
Galatians 6:7-9
7 Do not be deceived; God is not mocked, for whatever a man sows, that he will also reap. 8 For he who sows to his own flesh will from the flesh reap corruption; but he who sows to the Spirit will from the Spirit reap eternal life.9 And let us not grow weary in well-doing, for in due season we shall reap, if we do not lose heart.
Hope this helps,
God Bless