Judas did not leave the last meal.
Luke 22:
[14] And when the hour was come, he sat down, and the twelve apostles with him. [15] And he said to them: With desire I have desired to eat this pasch with you, before I suffer.
[16] For I say to you, that from this time I will not eat it, till it be fulfilled in the kingdom of God. [17] And having taken the chalice, he gave thanks, and said: Take, and divide it among you: [18] For I say to you, that I will not drink of the fruit of the vine, till the kingdom of God come. [19] And taking bread, he gave thanks, and brake; and gave to them, saying: This is my body, which is given for you. Do this for a commemoration of me. [20] In like manner the chalice also, after he had supped, saying: This is the chalice, the new testament in my blood, which shall be shed for you.
[21] But yet behold, the hand of him that betrayeth me is with me on the table. [22] And the Son of man indeed goeth, according to that which is determined: but yet, woe to that man by whom he shall be betrayed. [23] And they began to inquire among themselves, which of them it was that should do this thing. [24] And there was also a strife amongst them, which of them should seem to be the greater. [25] And he said to them: The kings of the Gentiles lord it over them; and they that have power over them, are called beneficent.
Judas was most definitely present when Christ instituted the Eucharist.