If you wish to reason with the JWs, your primary resource should be "Reasoning from the Scriptures" (thanks to yknot for the links she posted a couple of days ago in this thread - check there for other downloadable resources). JWs will refer to this book as primary resource when any question arises that they can't answer by themselves. Your JWs will surely discuss something along these lines.
Here's what the Reasoning book says on John 6:53, 54 :
Does John 6:58,54 indicate that only those who do partake will gain everlasting life?
John 6:53, 54: "Jesus said to them: 'Most truly I say to you, Unless you eat the flesh of the Son of man and drink his blood, you have no life in yourselves. He that feeds on my flesh and drinks my blood has everlasting life, and I shall resurrect him at the last day!"
This eating and drinking would obviously have to be done figuratively; otherwise the one doing it would be violating God's law. (Gen. 9:4; Acts 15:28,29) However, it should be noted that Jesus' statement at John 6:53, 54 was not made in connection with the inauguration of the Lord's Evening Meal. None who heard him had any idea of a celebration with bread and wine used to represent Christ's flesh and blood. That arrangement was not introduced until about a year later, and the apostle John's report about the Lord's Evening Meal does not begin until more than seven chapters later on (in John 14) in the Gospel bearing his name.
How, then, can a person "eat the flesh of the Son of man and drink his blood" in a figurative way if not by partaking of the bread and the wine at the Memorial? Notice that Jesus said that those thus eating and drinking would have "everlasting life!' Earlier, in verse 40, when explaining what people must do to have everlasting life, what did he say was the will of his Father? That "everyone that beholds the Son and exercises faith in him should have everlasting life." Reasonably, then, 'eating his flesh and drinking his blood' in a figurative sense is done by exercising faith in the redeeming power of Jesus' flesh and blood laid down in sacrifice. This exercising of faith is required of all who will gain the fullness of life, whether in the heavens with Christ or in the earthly Paradise.