Did they tell him 17% of his hert functioning Or did they tell him that his Ejection Fraction (EF) is 17%. There is a big difference. I've yet to her of a patient being told that only 17% of their heart is functioning.
The EF tells how much blood the heart is pumping out. 17% (mine was at 11% when I got sick) can be dangerous for several reasons. First it puts a patient at high risk for stroke. Second it puts them at high risk for Sudden Cardiac Arrest.
I pray your friend is doing better...
Jerzeycate
This link with pictures may help explain what's happening with your neighbor's heart.
http://myheart.net/articles/heart-blockage-explained-with-pictures.