iOS 8.4 update is causing iPad to restart (loop)

My dad tried to update his iPad to iOS 8.4 but the update never finished/installed successfully. Now the iPad just restarts over and over again. When I press the lock and home button together, it brings me to a black screen with the Apple logo and eventually goes to a white screen that says "Slide to upgrade." When I enter the passcode and try to continue the update, the iPad just restarts again. We were unable to get the iPad into recovery mode because we cannot turn off the iPad (it just restarts when we try to turn it off). When we connect the iPad to iTunes, it asks for the password and when we enter it it just restarts over and over again. Can anyone help? Thanks in advance.