Satan knew right away, that is why he prompted Herold to destory the children.
Matthew 2 with Rev 12, Herold represented by the red dragon.
Your stongest argument is the fact that the demons recognized who Jesus was right away when they saw him, I doubt that the prince of demons would be less observant. Your friend can argue that Satan did not know what God’s plan was concerning the cross, but that does not mean Satan did not know who he was.
Matthew 8:29
And, behold, they cried out, saying, What have we to do with thee, Jesus, thou Son of God? art thou come hither to torment us before the time?
Mark 5:7
And cried with a loud voice, and said, What have I to do with thee, Jesus, thou Son of the most high God? I adjure thee by God, that thou torment me not.
Luke 8:28
When he saw Jesus, he cried out, and fell down before him, and with a loud voice said, What have I to do with thee, Jesus, thou Son of God most high? I beseech thee, torment me not.
In his temptation, Satan knew exactly who Jesus was,
Matthew 4
The Temptation of Jesus
1 Then Jesus was led by the Spirit into the desert to be tempted by the devil.
2 After fasting for forty days and forty nights, he was hungry.
** 3 The tempter came to him and said, If you are the
Son of God,** tell these stones to become bread.
Note: apeal to Jesus as creator.
4 Jesus answered, It is written: ‘Man does not live on bread alone, but on every word that comes from the mouth of God.’
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5 Then the devil took him to the holy city and had him stand on the highest point of the temple.
6 If you are the **Son of God,** he said, throw yourself down. For it is written:
‘He will command his angels concerning you, and they will lift you up in their hands, so that you will not strike your foot against a stone.’
7Jesus answered him, It is also written: ‘Do not put the Lord your God to the test.’
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8 Again, the devil took him to a very high mountain and showed him all the kingdoms of the world and their splendour.
9 All this I will give you, he said, if you will bow down and worship me.
10 Jesus said to him, Away from me, Satan! For it is written: 'Worship the Lord your God, and serve him only.'
11 Then the devil left him, and angels came and attended him.