How to Understand Jesus' Explanation of His Words in John 6

[Nita]

… He explicitly said his words were spirit and life, the flesh avails nothing. THAT is what he’s talking about; ‘words’ not ‘flesh’ - but those who didn’t believe didn’t understand his explanation."

"No, Jesus had not just commanded his disciples to eat his flesh; he had commanded them to accept his words…

I would use Our Lord’s words at the Last Supper, where He again tells the apostles to “eat His body” and “drink His blood”, to prove to your friend that “flesh” and “blood” in those phrases do not mean Jesus’ “words”.
At the Last Supper Jesus takes physical bread in His hands and says:
“THIS” (bread, not words) is My Body; eat it.
“THIS” (wine, not words) is My Blood; drink it.

The phrases are not metaphors meaning listen (“eat/drink”) to My words (“flesh/blood”). The Last Supper makes it crystal clear that those phrases mean they are to physically eat and drink what appears to the senses to be material bread and wine, but “IS” in reality Jesus’ Body and Blood.

This IS My Body;
This IS My Blood;

 

[Chesster]

“It is the spirit that quickeneth: the flesh profiteth nothing.”

“what means ‘the flesh profiteth nothing’? It profiteth nothing,
but only in the manner in which they understood it. They indeed
understood the flesh, just as when cut to pieces in a carcass, or
sold in the shambles; not as when it is quickened by the Spirit.”

• St. Augustine

Thus it profits nothing to eat mere dead flesh, such as is sold
in the market, separated from the spirit, that is from the soul
and divinity of Jesus Christ. It was in this sense the Capharnites
understood the words of Jesus. But on the other hand, the
sacramental manducation of his flesh profiteth much. The error
then of the Jews consisted not in thinking that our Lord wished to
give us his flesh to eat, but in conceiving that we were to eat it,
as we eat flesh sold in the market. - St. Alphonsus Liguori

“The words that I have spoken to you, are spirit and life.”:

NOR is there anything contrary to the tradition of the Church in the
word of the Lord saying to the disciples, who seemed scandalised: The
words that I have spoken to you are spirit and life (John vi, 63). He
did not thereby give them to understand that His true flesh was not
delivered to be eaten by the faithful in this Sacrament, but that it
was not delivered to be eaten in the way of ordinary flesh, taken and
torn with the teeth in its own proper appearance, as food usually is;
that it is received in a spiritual way, not in the usual way of fleshly
food. - St. Thomas Aquinas