Hi, welcome to the forum.
It's hard. It has to come from within themselves. There's too much fear going on in their head; fear of losing their eternal future life, fear of apostates, fear of Satan, fear of losing their friends and family that is in.... fear.
This means that even if they are subjected to factual, truthful statements and findings that expose the WBTS, they will push it away; almost literally put their fingers in their ears and sing la-la-la-la.
I think I opened up to the possibility that they were wrong because everyday life made me have to skip a few meetings, and I eventually managed to look at things 'from the outside'. Only short glimpses at first, but it was eventually enough to make me finally curious enough to do some research.
That said, I vividly remember an illustration a CO gave in an assembly talk; this was so horrible that even though I was still in at the time, it made me see things differently. So it can be done. Then again, I was never a 'super-witness'. So I guess I already had the distance needed to be able to actually think for myself about the content of the illustration.
- If at all possible, I think it takes time. You can't just show them something out of the blue and expect them to leave instantly, no matter how factual and true that something is. But... people are different.
Some here have some experience in this, but I'm not sure how successful they've been.