From my understanding of your current problem, would suggest the following in order of preference.

Option 1.
Hack the affected area and lower the aircon piping. The water from the aircon condensation is drained under gravity. If there is not enough gradient, the aircon will not function efficiently.
[The method proposed by the plaster ceiling guy will not work as these will be water trapped inside the flexible pipe. It will be a matter of time before water starts leaking]

Option 2.
Provide the box out to allow the top of the aircon blower unit to go above the ceiling level. It may not be aesthetically pleasing, but have seen this method being used when there is a window opening just below the aircon blower unit.

Personally, for durability and aesthetic, I would go for option1.