What can I add to boiled lollies to stop them from melting in humid weather?
To be precise, the lollies are not melting. They are absorbing water from the air.

If you're really just crushing the lollies (not melting them and re-casting them into sheets of "glass") then you're accelerating the absorption process by increasing the surface area. 

Does it need to be out in the open for five days? I'm surprised the gingerbread hasn't gone soggy (actually, I'm surprised it's not been eaten). I would consider putting it in an airtight box until it's ready to serve.

Otherwise, you might be able to make the windows last longer by reducing their water content. Put the crushed lollies in a pan, bring them to the boil and let them simmer to lose more water. Use a sugar thermometer to find the necessary stopping point. Be careful not to burn the sugar. Then pour onto a flat greased surface, to solidify into "glass".