IQ" is a very broad and full of misconceptions. While it may be the most popular form for measuring intelligence, it has been debated a lot of it's scientific properties and accuracy.
In the case you explain,the individual in question has high understanding of mathematics and logic and low emotional intelligence. If we are following the famous interpretation of Gardner's of intelligence with the Theory of Multiple intelligences, there is no direct correlation between the two; but, by logic and human limitations, not everyone can have high intelligence in all its different encarnations. It can be perfectly normal to be considered a genius in some area but "average" in others.
As we understand this, it is no secret that an emotional unstable person may have difficulties getting their thoughts in order and may have poor performance by stressful factors formed by their disruptive psyche. I feel obligated to note that to be unstable by your emotions, you need to first recognize and acknowledge them, and to do that you need to develop some tools provided by emotional intelligence.
And as well, there are many theories that as we grow old if not maintained and reinforced, our intelligence may start to deteriorate, and that many factors that increase this process are tied to psychological stress.
There is definitely endless variables that can not be determined by the lack of details in the case proposed, but this is just a broad interpretation and should not be taken as absolute truth and be used to explain every person that may fit this case.
If you want to research more on your own, the previously mentioned Garner's Multiple Intelligences: New Horizons in Theory and Practice is a must read to understand one of the most prominent theories of intelligence.
Emotional intelligence can be studied in much more detail in the work of Goleman in Emotional Intelligence.
And while it can be a heavy read, any serious book on neuroplascity can be helpful to understand the ever changing brain and it's functions and "agility". For example, Neuroplasticity by Mohed Constandi.
Hope this answer helps you.