First, ask them where the Bible says this. They will probably point you to Rev. 22:18-19–“I warn everyone who hears the prophetic words in this book: if anyone adds to them, God will add to him the plagues described in this book, and if anyone takes away from the words in this prophetic book, God will take away his share in the tree of life and in the holy city decribed in this book.”
Then ask them what the author meant by “this book.” They will probably say, “The Bible.”
Ask where the verses claim that by “this book” John meant the Bible. They will not be able to, since the word “Bible” is not used in the text. Then point out that John couldn’t possibly have meant that because:
Context. John refers to “this book” also as “this prophetic book” which means it was only the one he was writing at the time, that is, Revelation
Revelation was written about 100 A.D. but the Bible was not put together in its final form until late in the 4th century
Now point out that they themselves have added something to their religion that is not in the Bible. They have added an interpretation that is not supported by the text. Worse still, they are using this interpretation to condemn people to hell.
I would then take them to Matthew 7:2–“For as you judge, so will you be judged, and the measure with which you measure will be measured out to you.” In other words, they have just condemned themselves, for they are doing exactly what they claim a Christian will be damned for doing: adding things to their religion that are not in the Bible.
Hope that helps!