How do I unlock my wife's iPad

It depends upon the kind of lock:

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - https://support.apple.com/HT204306

Harder:

iCloud: Activation Lock - http://support.apple.com/kb/PH13695 - This is a security feature so only legitimate (i.e., not stolen) redistribution of devices is possible.

Turn off Find My iPhone Activation Lock - https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it. In your case you will need to take a death certificate to an Apple Store, probably also the original receipt too. Check with them what they will require, it may depend upon who actually bought it.