What to know
First, remember what many Muslims are taught: that Christians worship God the Father, Mary the mother, and Jesus the Son. This unholy "Trinity" is rightly rejected by Muslims.
Before you discuss what the Sonship of Christ means, make sure to clarify what it doesn't mean!
What to say
The Bible claims an amazing paradox – that Jesus was equal to God, and that he was subservient to God, as a son is to the Father. What does this mean? (Note: As you try to express this, remember that your friend's native language isn't English. Although this post is written in high-fluency English, it's important to use vocabulary and sentence-length that's appropriate for your friend. Don't try to copy and paste the phrasing used here...say this in your own words, using vocabulary your friend can understand. This might mean looking up synonyms for words, or using Google Translator to send possible translations of the word you're looking for in your friend's first language.) Begin by stating Gospel-Truths:
Followers of Jesus believe that God is completely above us. A human can't exist in more than one form at once. But for God, nothing is difficult or impossible. The Word of God testifies to this.
God's wisdom is also above ours. He chose to exist in more than one form at once, and to interact in a relationship format of Father and Son. In order to understand this, let's pause to look at these terms...
What does "Son of God" mean? See Luke 1:35 and its surrounding chapter: "The Holy Spirit will come on you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God." Jesus was called God's Son because he had no human father; a recognition of his miracle birth. Muslims will (rightly) suggest the same should also apply to Adam, in that case. We see an echo of this in Luke 3, where Jesus's genealogy (interestingly enough, through Joseph) is recorded, ultimately leading to a statement that Adam is "the son of God." Explore this concept further in 1 Corinthians 15. Adam and Jesus are unique in that they have no human fathers; however, Jesus is elevated high above Adam. The Bible says clearly that Jesus existed at the beginning of the world; he didn’t come into existence when he was born of Mary (see John 1). Read the accounts of his birth, and then John 7 and John 8 together and discuss.
Note also the recurring question regarding where Jesus was from, seen often in Scripture. Explore his claim that he is "From above" in John 3 and John 7.
Now explore the obedience of Jesus: One of the most central aspects of Sonship that Jesus showed. In Hebrews chapters 2 – 4, we see this discussed. Jesus chose to submit to the weakness of humanity. This includes physical pain and even death, but it also includes emotional exhaustion and the reality of temptation. Jesus chose to not only experience temptation, but to experience it at such a real level that he “suffered” when tempted. Yet Jesus chose submission to the will of God – his Father in heaven, living free from temptation – every single time. He lived a perfect life. He did this for us, to give us a perfect image of surrender. At the Garden of Gethsemane, moments before his arrest, we see this most clearly. Jesus knew he was about to be arrested, and he knew that no one could take his life from him. Read Matthew 26:52-53 and John 10:17-18. Yet he submitted to death – he drank the cup that God gave him. Jesus is the perfect image of submission. It’s his Sonship —his “weakness” and pain—in which he is subservient to God the Father. But it’s his divine identity that makes he himself worthy of worship – the perfect image of the holiness and power and love of God. In the book of Revelations, we see him worshiped as the Lamb who was slain. Let’s read the Bible together to understand the nature of God as Father, Son, and Spirit, and the worship that they are to receive. Consider Nabeel Qureshi's examination of this aspect of Sonship in this video.
Jesus is the Son of God, but he more frequently referred to himself as the Son of Man. This should actually be considered the more controversial term! This phrase refers back to Daniel 7, where the Son of Man is seen in a glorified state. Encourage your friend to read this book and to consider through the significance of this term.