What your colleague asks is a valid question. Quran is not just the number 1 source, but it is the first and last source of all religious guidance. Those who have faith in God and trust His message delivered through Prophet Muhammad AS, and have read the whole Quran know this very well. Those who have doubts in God and wish to honor and accommodate their idols including their religious scholars want to add other sources from their past generations, who can dictate them laws in addition to God’s laws. They come up with flimsy excuses against Quran such as Quran does not cite how to offer Salat, Obeying Messenger requires following hearsay based Sunnah, Tawatur and established history are divine etc.
When Prophet Muhammad was delivering the Quran, God’s opponents wanted him to compromise and accommodate their religious sources:
[68:9] They wish that you compromise, so they too can compromise.
Unfortunately, scholars today knowing the truth do compromise and end up upholding false religious doctrines. Javed Ahmed Ghamidi is one such example. Knowing full well that Quran is a complete, perfect, and fully detailed Scripture, detailing all religious knowledge and explaining the divine laws in detail, and containing end-to-end Sharia, he upholds and justifies the false doctrine of Sunnah, which has no basis in Quran. He has written 7 totally arbitrary and baseless principles of Sunnah in his Book, which people following him blindly honor as a divine code, while treating Quran as a secondary source.