Leaning cinder block fence caused by tree, what's the best way to fix this?
Honestly, this looks like the entire wall is affected by general differential settling and it looks pretty serious judging by the adjacent fencing.  If there is ANY chance of the wall falling onto your neighbors - like a child going to get that soccer ball, you will need to demo the wall, grind the stump completely, and compact the soil.  A new masonry wall would need to be engineered by a licensed professional, and installed with the engineer's specs for foundation and compaction.  In a more remote location, you might grind the stump, and then repoint the masonry joints.  This would be a cosmetic fix, but, now that the stump is removed, the roots under the wall should no longer grow, but may even decompose to their original condition bringing the wall more plumb.  But if we squash the neighbor's soccer toddler, the higher price of a new engineered wall will seem small.  Your neighbor should share in the cost of the fix, too.  If not, he shouldn't complain when you put back a simple wood privacy fence.  There may be other factors, such as warranty issues from the original builder and homeowners association requirements.