It can be difficult to tell the difference between INTJ and ISTJ. Various tests had me pegged as ISTJ, at first. Part of the problem is that the tests' N vs S questions can be really quite stupid: "Do you prefer abstract theory or real factual details?" and variations on a theme. N is not a preference for "abstract theory" nor is S a preference for detail work. It's an abuse of the word "preference" in the MBTI context.
See, I "prefer" real factual details, because in real life, I know that if I don't have the real factual details (and it doesn't need to be many, just a good representative sample), then all of the various theories, thoughts and ideas floating around in my head are so much useless fluff. But really, I process things in terms of functional patterns that are not strongly anchored to specific details. I "memorize" functionality, not lists of detail. That's Ni. I process and match patterns, not specific events. That's Ni.
What one "prefers" in an MBTI context really means what one "uses"; it means how one actually processes incoming information, without regard to what one likes.
So, if your friend processes things "to the letter" as a matter of habit, and you don't see him regularly gain bizarre insights by noting how X works just like Y did, except for a couple of details, so he can use his understanding of Y to work with X and develop a deeper understanding of X, then your friend is probably ISTJ, regardless of how quirky and independent he might be.
Similarly, no matter how conventional, stick-in-the-mud, mundane, and boring your friend might be, if he can just look at a problem that looks unsolvable, solve it in his head, and then gets a bit frustrated as it takes forever to explain his solution and convince others of his solution, then he's likely to be INTJ.
MBTI doesn't indicate one's attitude, beliefs or emotional state. MBTI indicates how one processes incoming information and makes decisions based on that incoming information. The actual conclusions and attitudes of individuals of a given MBTI type can be remarkably disparate. The similarities you will see between individuals of a given MBTI type are far more functional than specific.