They missed a spot.
Here is John 1:3 in their own NWT:
All things came into existence through him, and apart from him not even one thing came into existence.Do you see it? Everything that came into existence did so through him. Everything. So Jesus didn't "come into existence."
What do we do with this? Ask a JW to read this verse from his Bible. On a piece of paper, draw a box. Label it like the one below.*
Ask the JW to put an X where Christ should go. He'll want to put in in the box of thing that were made. But the verse says Christ made everything that was made, so he can't have been made.
He'll want to say it means "everything else" that was made. But that's not what the text says. Don't let him do it. Remind him, "apart from him not even one thing came into existence."
Push him on this for as long as you can. It's unlikely he's going to fall on his knees and declare Jesus is God right there — he might, but probably not.
What we're hoping for, though, is that this will stick with him, bug him, work at him. If he sees the inconsistency here, he might begin to see it elsewhere. And one day he really may fall on his knees and declare Jesus is God.
*I wish I could take credit for this; I can't — I read it somewhere. I wish I could credit that source; I can't — I can't find it. But I want to openly acknowledge that I didn't come up with this.
Was the Cross Just?