Mat. 20:8-16, KJV shows Jesus's response to those (Jews) who murmured at the lord of the vinyard. In verse 13, notice that his answer:
But he answered one of them, and said, Friend, I do thee no wrong: didst not thou agree with me for a penny? (My emphasis)
Again, in Matthew 22:12-13, the king who made the marriage for his son, after seeing a man which did not have a wedding garment, said the following:
And he saith unto him, Friend, how camest thou in hither not having a wedding garment? And he was speechless. Then said the king to the servants, Bind him hand and foot, and take him away, and cast him into outer darkness; there shall be weeping and gnashing of teeth.
And, of course, after being kissed by Judas, Mat 26:50 again repeats as follows: And Jesus said unto him, Friend, wherefore art thou come? Then came they, and laid hands on Jesus, and took him.
Each of these instances reveals a special application of the word, "Friend". Remember, just a very short time earlier, Jesus, who knew Judas was to betray Him, had just washed the feet of Judas, along with the other disciples. Verse 13:10-11 is very instructive concerning that feet washing:
Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all. For he knew who should betray him; therefore said he, Ye are not all clean.
Then, immediately after explaining what He had just done unto them, verse 21 reveals:
When Jesus had thus said, he was troubled in spirit, and testified, and said, Verily, verily, I say unto you, that one of you shall betray me.
One of who? This is clearly one of the special applications of the use of the word, ἑταῖρε, or Friend. Notice that most versions use an upper case "F" here, so this is indeed special. Remember, Abraham was the "Friend of God", forever (2 Ch 20:7) Abraham was the father of all Jews, including David who sheds MUCH LIGHT upon this use of the word, "Friend" in the 35th Psalm, in verse 14:
I behaved myself as though he had been my friend or brother: I bowed down heavily, as one that mourneth for his mother. (My emphasis)
So Jesus washed Judas' feet, as Psalm 109:4 directs:
For my love they are my adversaries: but I give myself unto prayer.