From your description is sounds like a long term exposure situation - short term there would be no issue.
It seems to me that you have two problems - both ends of the light spectrum, UV and IR. UV can damage the polariser film. Luckily the window (if it is definitely Polycarbonate) has good UV blocking properties.
At the other end of the spectrum, IR is more of a problem. LCDs don't like very high temperatures, initially you get a shift in contrast, then, at higher temperatures, they go black. You need to keep the LCD reasonably cool - if it really is in direct sunlight then the customer needs to arrange for some level of ventilation (or at least, air circulation) over the display. This is going to depend on the size of the enclosure, sealing, ambient temperature, etc. Worst case, some sort of thermally conductive pad to an aluminium chassis part behind the display might help.
One final problem might be moisture / condensation, but that will probably affect other things in the enclosure too.