When John says “Do not add to this book,” what is he referring to and how do I answer a non-Mormon’s inquiry?
How you should answer depends entirely on what you think this person might be attempting to understand. If you feel your friend is sincerely looking for an answer versus simply trying to attack your faith, different answers are warranted. Only you can judge that. Plus it has been my experience that people interpret those scripture differently and therefore different answers are required. You did not include how your non-Mormon inquirer understood it themselves. So I am going to assume a sincere inquirer and I am going to cover the two understandings I have found to be the most common.
First lets reference the scripture I believe you to be talking about.
18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
The first interpretation we commonly get is the idea that God has said no more words will be given That God is done speaking to man and giving His word. That anything else is an addition by man. This understanding makes it so that the Book of Mormon and other modern revelations simply can not be true. There are a few problems with this understanding. First and foremost it does not accurately say that anywhere. It is a warning against man altering, adding, or subtracting from the Word of God. It says nothing about God stopping the revealing of His word.
Second we have an issue with another scripture in this case Deuteronomy 4:2
2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you.
It is the same scripture commanding men not to add or remove from the words God has given them. The only thing Deuteronomy lacks is the penalty for doing so. So by the logic and reasoning that they use to say Revelation says no more word will be given and everything else must be false addition, then our Bible should stop at Deuteronomy. Most Christians will rightfully protest that the New Testament should be tossed out as an addition of men. Yet that is what their logic demands when applied to Deuteronomy.
In addition, according to our best understanding of when the Book of Revelation was written by John. He then after words wrote the Gospel of John and the Epistles of John, so the very Apostle who wrote those verses did not understand it that way. He added more scriptures after Revelation and I think we can trust his understanding.
The second understanding is that God will not allow His word to be altered. Since the LDS faith believes that truths were lost because it was altered, if God said He will not allow His word to be altered then the LDS faith has a problem. However the logical wrestling one must do with those scripture to reach that conclusion makes my head hurt.
Have we ever seen anywhere in the scripture where God gives a command and a punishment for breaking that commandment, and then says “Never mind I will simply make it so you can’t break my commandment after all?” I have never found that anywhere else in the scriptures, but that is what they require in this case. That understanding make no sense whatsoever based on what the verses say. It is much more logical to take those verses and assume that somewhere someone did add to and did take away and did get the plagues added to them and did get their name taken out of the Book of Life, and the Holy City, etc. Otherwise the verses are wasted space. Of course with that assumption a restoration of things like the LDS faith teaches is both logical and necessary to restore what was altered.
Anyway Robert, I hope I was able to help you and give you things to ponder as you prayerfully ask the Lord how you should answer the inquirer you have.