Apple ID cannot be used to unlock this iPad

Did she or someone else erase your device without signing out of iCloud first? If so, that why the iPad is activation locked. She can turn activation lock off only by entering the correct Apple ID/password combination. The correct combination is the one used by the person setting it up prior to erasure. If she can enter this combination, then good. 

Note that Security and your Apple ID https://support.apple.com/en-us/HT201303 says “without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account.”

 If the combination mentioned above cannot be entered, then the iPad is useless unless there are extenuating circumstances (e.g., she has the receipt for your purchase of the device with its serial number on it or a court order instructing Apple to assist in the provision of access to a decedent’s information from the deceased person's accounts--(see How to request access to a deceased family member's Apple accounts https://support.apple.com/en-us/HT208510) If you have either, contact Apple support using one of these options. 

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help  https://getsupport.apple.com/. Go through the options to find the best support option.

Somethings to try. Worst case, she will end of where she is right now.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.      

So, the iPad is activation locked, right?

You are very welcome. I hope she can recover her passcode as Kappy suggests.

Perhaps the problem is forgetting her lock screen passcode, not the Apple ID.

Forget Passcode or Restore Device

1. If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled
2. How to erase your iOS device & then set it up as a new device or restore it from backups

For help with the above or later models including iPhone 7, 7s, 8 or later, iPad Pro 11" and iPad Pro 12.9" models please refer to If your iPhone, iPad, or iPod touch won't turn on or is frozen.

If the above doesn’t work for you, then:

Follow these steps to place your iOS device into recovery mode: iPhone DFU mode explained, and how to enter DFU mode on your device.

If all fails then make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. If you did not purchase it from Apple then you will need to look to the vendor for service.

If the problem truly is with the Apple ID, then she must take the iPad, her original, dated purchase receipt that shows she is the original owner, and a valid picture ID to a local Apple Store where they may (of may not) be able to help.

Activation Lock Problem

If you purchased the device second-hand, then it is useless without the Apple ID and password to which it is locked. If you don't have it, can't remember it or cannot get it, then the device is useless to you. Try to get your money refunded, if you can, or take it to an Apple store for an exchange provided they will help you. If you are the original owner, then take the device, a copy of your purchase receipt and identity proof to an Apple retailer where they may be able to assist you. For a deceased owner, you will need a certified copy of the Death Certificate, the device, proof of inheritance and proof of your identity. Take all to your local Apple Store for help. Note: there is no guarantee that anyone can gain access to the device.

About Activation Lock on your Mac - Apple Support

Activation Lock - Turn off Find My iPhone

About Activation Lock on your Apple Watch - Apple Support.