Hi Groovy yank,
Is it a 'true' mini fridge? as in it uses a compressor and refridgerant based system? or is it one of those 'mini cool boxes' that you connect up to a little power supply and it 'fans' the inside of the box?!
either way, yes you can leave it on - but if it's one of the 'mini cooler' box types you'll probably find the internal fans will give up in a comparitively short space of time - i'm talking months here of course. A true 'refridgerator' will last much much much longer of course.
Although minimal, will his employers allow it's use? over the course of a year, it will cost maybe £20-30 or so if it's run continuously (i'm not sure of the exact calculation though, depends on it's power supply ratings and unit of electricity prices at the business!).
I operate a switch it off policy at our company - so as I'm usually the last to leave, I switch off anything I don't want left on and includes a member of staffs 'mini cooler box' on her desk!
Cheers! DtM! West London & Slough UK!