Ask her what SHE would accept as proof, then explore the possiblility of fulfilling that requirement.
If she says she would have to see a watchtower from 1890, for example, help her to appreciate that finding an original hard copy is going to be pretty time-consuming and expensive. Does she insist on pursuing this, or would she accept scanned images of the pages?
..and YES, scanned page images can be counterfeit. Let's not pretend they can't be. What we need to look at is the possible motives for forgery of a professional like, say, Research Applications International. If he produced forged documents his business would be worth zero.
On the other hand, if the TRUTH would prove those other quotes are based on distortions and lies, why doesn't the WATCHTOWER make the genuine documents available to refute the liars?
If you're in NYC, maybe you can arrange to visit the Bethel Library for some personal research.
Do any UN officials have a Bethel Library Card?