Removing trusted Phone number
From Apple KB - Two-factor authentication for Apple ID

You can update your trusted phone numbers by following these steps:
Go to your Apple ID account page.
Sign in with your Apple ID.
Go
to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number
and enter the phone number. Choose to verify the number with a text
message or an automated phone call, and click Continue. To remove a
trusted phone number, click x icon next to the phone number you want
to remove.

Presumably, if this is someone else's iPad, they will have to do the removal.
Alternatively, you may be able to see &amp; remove their device from the Devices list on your Apple ID page. I'm not sure how this would affect future trust, as it wouldn't necessarily clear the number from their account.