that won't work on the cladding, you can apply the coating over the aluminium no problems, you can also do this with the polystyrene sheet, it would work prefectly,
The issues is when the exterior cladding gets damaged which allows access to the flammable material below.
you could pretty much pull the cladding off, remove 10mm of the polystyrene, apply a product like bostik fire seal, It will have the same effect, any damage to the board or if the sealant fails the building is at risk.
They could always install a sprinkler system over the whole building, although it will most likely be cheaper to replace the panels with fibre cement board, or the same product with a fire retardant core.
