Well, first the word Eucharist is Greek and it means, if I’m not mistaken “Thanksgiving”. That is giving thanks to God. Ok, so given that they didn’t use the word Eucharist which I believe was first used by St. Justin Martyr in the 2nd century to describe it to the Roman emporer, they discuss it as the “breaking of the bread”.
Let’s look at Luke’s Gospel and the description from the disciples who were on the road to Emmaus and how they didn’t recognize Jesus until he said Mass (yes that’s what he did!)…
usccb.org/bible/luke/24
29But they urged him, “Stay with us, for it is nearly evening and the day is almost over.” So he went in to stay with them. 30And it happened that, while he was with them at table,
he took bread, said the blessing, broke it, and gave it to them. 31With that their eyes were opened and they recognized him, but he vanished from their sight. 32Then they said to each other, “Were not our hearts burning [within us] while he spoke to us on the way and opened the scriptures to us?” 33So they set out at once and returned to Jerusalem where they found gathered together the eleven and those with them 34who were saying, “The Lord has truly been raised and has appeared to Simon!”q 35Then the two recounted what had taken place on the way and
how he was made known to them in the breaking of the bread.
See what happens to them. Those disciples are all of us! We are blind really and full of sorrow and grief, etc. But thru the Eucharist, which is how Jesus stays with us physically, we can see. And we are to do exactly what those disciples did …run and tell everyone. The Mass is about going forth to spread the news of what we learned and experienced.
So if your friend agrees the Last Supper (first Mass btw) was about communion, then they would have to agree that when they are described as recognizing him in the “breaking of the bread” that it is the same thing. Therefore when the early Christians gathered it was for the breaking of the bread.
See what St. Paul says about it a bit earlier in Act 2:42…
42 They devoted themselves to the teaching of the apostles and to the communal life,
to the breaking of the bread and to the prayers.
Also St. Paul writes in 1 Corinthians 10:16 - 17…
16The cup of blessing that we bless, is it not a participation in the blood of Christ? The bread that we break, is it not a participation in the body of Christ?l 17Because the loaf of bread is one, we, though many, are one body, for we all partake of the one loaf
In 1 Corinthians 11…
20 When you meet in one place, then, it is not to eat the Lord’s supper…
and goes on to describe not recognizing that it IS the body of Christ (eh hmm - the real presence!

) The first part here (23 - 25) describes the tradition being handed on that is transubstantiation…
23* For I received from the Lord what I also handed on to you, that the Lord Jesus, on the night he was handed over, took bread, 24and, after he had given thanks, broke it and said, “This is my body that is for you. Do this in remembrance of me.” 25In the same way also the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.”
26For as often as you
eat this bread and drink the cup, you proclaim the death of the Lord until he comes. 27Therefore whoever eats **the bread
or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord. 28A person should examine himself, and so **eat the bread *
and drink the cup. 29For anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.
There are plenty of apologetic works out there that describe this much better than I. I’m sure somewhere this is all put together in a document or book. These are just a very few of the scriptures that describe it that I’ve come across and have heard from others. I encourage you to study these scriptures closely prior to any discussion with your friend. Also helping your friend understand The Mass …what is is and what goes on and why, will be helpful.
But I can tell you, that these (and many others) were key scriptures in my study of the faith whereby I found The Catholic Church to be the true Christian church! Never know, you may be witnessing to a future convert
