You let them decide.
You let them ask you.
Then you say, "What if .....the truth about WT leaves you depressed and pained with regret and lost. We both know there is a lot to lose"
Then you don't tell them outright anything. It will be so tempting when they ask "like what?" Think about it deeply, your own experience, what takes months to research in person is powerful and visceral. If summarised in 3 minutes, it can be ignored and forgotten in 3 minutes. I've done it myself, a nuclear explosion of anti J.w logic and evidence, presented too easy to digest, too succinctly, they can hit 'ignore' in their mind for those two minutes with ease, distracted with an excuse they are desperate to fire back, so much so they ignore your point entirely. The ministry has taught them to argue well, not reason well! If you could easily reason with a JW..... there would be no JW's!
Point them into the direction of their own research. Let them feel the visceral emotions of discovering TTATT, because once you do, you can never go back. It's impossible! Its like trying to pretend Santa exists at 53 yrs old.
"You can only know your own motives, not the motives of anyone telling you anything. You don't even know mine, though I know they are good, I seek the TRUTH only, you must do the same for yourself! Don't trust anyone, including me! I can save you time by pointing you in the right direction for data and topics worth exploring, but only if you wish. If you prove me wrong, ill come back to WT."
Be careful to think you can sling information at them and de-convert them. I have attempted this, they have answers for everything and a god that can magic any answer. They have already spent years, decades ....developing cognitive dissonance, your reasoning may dent their armour, but they self constructed that armour with the motive to protect their own beliefs. They have invested so much time, money, life into this. They have made marriage, child, career, family, future decisions bases on these beliefs....they are invested HARD. For years they have ignored their OWN minds, ignoring yours will come easy.
This is all achievable however it requires different approaches for differing people. I have helped several people leave, all have been wildly different. Only theme was.... let them do it.
Lastly, be careful what you wish for,....I had two close friends who insisted I tell them why I was leaving the JW's. They both went into a mega depression. One came out more brainwashed than ever. The other, he remains very unwell and depressed 15 years on, both still J.w's.