How to delete an app stuck in update mode?
Have you tried deleting Groupon, or any other problematic app, directly from your device?

Hold your finger down on the app icon.  When it wiggles, tap the delete button.  After confirming and deleting the app, download it again from the App Store on your device.

I've had this same problem before.  I'm not sure what causes the problem but this has always solved the problem for me.

I'm not sure why iTunes won't let you delete the application.  Maybe this is because it first poles your device to see if it is updating.  In such a case, it may prevent the deletion from occurring.

I hope this helps.  If it doesn't, I'd like to know how the device responds when you try to delete the app from the hardware.