I just had a new thought about your desire to explain why it’s taught about the use of symbolism in the bible as opposed to taking all passages literally (especially John 6, and with that Mark 14: 22 - 25; Matthew 26: 26 - 28; Luke 22: 14 - 20; 1 Cor 11:23 - 26). If these are to be taken literally (and I believe they must be) then the only way is to explain them as symbolic speach. Also, from the example of being washed in the Blood of the Lamb (Jesus) is clearly speaking symbolically.
So I would venture the real question becomes, how do we know what should be accepted as literal or symbolic. That may be a bigger problem for a couple of reasons. First, is the problem of “Sola Scripture” which I believes handcuffs people to the broader reality of how God reveals Himself through Apostolic Tradition and a particular aspect of Apostolic Tradition, namely, how has the Church’s liturgical life help us to understand revelation. For example, the development of the Doctrine of the Trinity has its roots in the Church’s Celebration of the Sacrament of Baptism.
A second difficulty I see is a kind of contradiction of “Terms in Action” that comes from Sola Scriptura. That is the rejection of the teaching authority of the Church, but in effect, most Christians who call themselves Protestant ultimately refer to an authority out side scripture, namely how an individual(s) had interperted scripture, then taught others their interpertation. People accept this not realizing it is not really coming to an understanding of Scripture through guidence of the Holy Spirit, rather, its an affirmation of someone else’s opinion that they affirm, without any real guarentee that the original interpretation was anywhere near correct, and in such a case as the Real Presence, goes against the held belief of the Church (East and West) from the time of Christ and the Apostles until sometime around 1500 AD when the acceptance of the Real Presence was challenged by John Calvin.
Having said all that, I doubt really your discussion would change your friend’s mind, but trust in the Holy Spirit, and study using good Catholic sources.