It's technically not possible. Authorized Users never get a copy of the card. It's sent directly to you, to your address on the account. Even if they call for a replacement card or something, they won't have the information needed to "verify" your account (like last 4 of your social, or your DOB, or anything about you), but further, even if they could, the replacement card is also sent to your address.
As an authorized user, they can't change any info on the account, nor get a card shipped to them. They'll be on the account for a month or two, and then removed with no access to do anything on the account, nor any account information, nor the ability to spend any money.
We have NEVER had a single case of an AU getting a card and spending money on it, and we've been doing this business for over 5 years already. As we know from other tradeline companies, they haven't had any instances of unauthorized access of AUs to buyers' cards during all their business years either, because it's technically impossible. So you shouldn't worry about it happening.