My friend has a 2009 XLE that if he switches the lighting switch to Door, the light immediately comes on regardless of the doors being closed or open. When all doors are closed the light is still on, however the gauge cluster does not indicate a door is open or ajar. What could be the problem.

 

First, make sure it is on the right setting. I'll assume this one is done, lol!
Next is to check the switched at each door. There will be these little plunger like switches in each door jamb that when the door is opened, they engage the activation for the light.
Over time these can wear down with normal use of the door opening and closing.
Easy to check is to ball up some masking tape and stick it to the switch. Do it for each door and then see if the light goes out. If it does, then it is just the process of elimination to figure out which one.
Maybe the door switches are the issue and maybe not but let's at least rule it out.