The test aren't all that accurate in determining one's mbti type. Also, nobody will posses 100% of any type's traits. A more efficient way of identifying your friend's main type is to thoroughly study the eight cognitive functions. For a very simplified description of them, look for my post here (ignore the irrelevant info):
https://answers.yahoo.com/question/index?qid=20141015142032AA2ra9O
INTJ: dominant Ni, auxiliary Te, tertiary Fi, Inferior Se
ISTJ: dominant Si, auxiliary Te, tertiary Fi, Inferior Ne
Introverted intuition dominant INTJ lives in a world of far-reaching implications, underlying meanings, patterns, connections, insights, and symbols.
Introverted sensing dominant ISTJ lives in world of nostalgia, sentiments, impressions, and memories.
So stereotypically...
Both the INTJ and ISTJ utilize execution-style, results-oriented thinking to make their plans and ideas come into a reality (auxiliary Te). They see the logical stepping stones they need to progress through in order to achieve a goal and strive for the practical application of their knowledge. They have relative difficulty expressing their emotions and forming personal value judgments independent of predetermined "codes (aka rules)" for behavior (tertiary Fi).
However, the ISTJ is better at recalling concrete sensory, tangible facts in their literal form than an INTJ, giving him a tight, precise grip of the physical aspects of reality (dominant Si). ISTJ is more prone to becoming sentimental and nostalgic. The INTJ is better at making associations and connections between events, which bestow upon him depth of understanding and foresight (dominant Ni). The ISTJ has a difficult time creatively imagining possibilities, thinking outside the box, and embracing change (inferior Ne). The INTJ is predisposed to feel overwhelmed with loads of sensory information, so therefore, has a comparatively hard time purely engaging in his five senses and will need to rely on his intellect for comfort (inferior Se). The ISTJ is past oriented, the INTJ is future oriented. The ISTJ has a conventional, traditional view of life. The INTJ has an unconventional, conceptual view of life.