Minus deductible up to policy limits.

Percentages come into play when limits with a liable party become an issue. If the liable party (in this case, probably the downstairs neighbor) only has liability coverage for 10% of the damage, then they will only pay each party 10% of their damages (they in this case being the insurance company of the liable party).

At that point, Mook might be better off going through his own personal property coverage on his renters policy (if he has that coverage).