“Can you please elaborate on your point of view regarding the Holy Spirit's illumination. I have always been taught that the Holy Spirit teaches us Scripture, through various different means, pastors, teachers, books/commentaries, etc.”
The locus classicus for this would be 1 Cor 12.
This would be a case of the Holy Spirit teaching believers indirectly via certain spiritual offices.
Usually, though, the term is used to denote something more immediate (i.e. direct) and individual. As C. C. Ryrie puts it:
“Specifically, the doctrine of illumination relates to that ministry of the Holy Spirit that helps the believer understand the truth of Scripture…illumination refers to the ministry of the Spirit by which the meaning of Scripture is made clear to the believer,” Evangelical Dictionary of Theology, W. Elwell, ed. (Baker 1984), 545.
For reasons I’ve already given, I regard this conception as erroneous and pernicious.
The Holy Spirit doesn’t teach us (in this sense.) Rather, the Holy Spirit makes us teachable.
“For example, recently a friend of mine was asked, 'Who teaches you God's truth', and he replied, The Holy Spirit thru the Bible.”
Yes, that quasi-charismatic view is very popular. It’s also a great way to make shipwreck of one’s faith.
“I guess this is taken from ‘When He comes, the Helper will guide you into all truth.’"
That may well be. But the Johannine passage is a promise to the Apostles, and not to Christians in general or the institutional Church.
“But of course, one must do a study to see what exactly this means, and not just read it as though it were written in english by John.”
Exactly. Your friend is taking the verse out of context.
Indeed, this becomes a viciously circular appeal. They appeal to a particular verse to prove their view of illumination, and then appeal to their view of illumination to prove their appeal to a particular verse—or vice versa!