The security of the AES secret itself I am writing an application, running on a server, where multiple users access data from a database which is AES encrypted with a master secret. The master secret itself is initially randomly generated, and then AES encrypted with a user-secret to yield a 'user-hash'. The master secret is never stored, but the user-hash is stored in a database.
When a user enters his user-secret, the user-hash is decrypted to temporarily (on the fly) yield the master-secret which is used to decrypt the data. The data is then send to the user.

If a user is deleted, the user-hash is deleted. He may have saved decrypted data, but cannot decrypt further data.

The question is: How safe is the master-secret in this constellation? If the database is compromised, an attacker has access to the user-hash and the encrypted data. If the attacker was a user, he may also have decrypted data. Will he then be able to break the master-secret?
If a user has a copy of both the encrypted and decrypted data, he is in a position to perform at least a known-plaintext attack. If users can submit arbitrary plaintexts for encryption, they can conduct a chosen-plaintext attack, which is stronger. In a chosen-plaintext attack, the attacker can submit any number of plaintexts and can retrieve the corresponding ciphertext.

All reasonable encryption algorithms are resistant to chosen-plaintext attacks, and AES is considered a reasonable encryption algorithm. Someone in possession of some (plaintext, ciphertext) pairs cannot encrypt or decrypt other messages (except sometimes messages derived from the known messages if a bad mode is used). In particular, no matter how many known plaintexts and ciphertexts the attacker is, he has no method to obtain the key that is better than brute force (trying all possible $2^{128}$ keys).

Do note that deleting the private secret does not delete the master secret. The master secret can still be recovered from backup copies of the private secret that weren't deleted, from other users' private secret, or from extant copies of the master secret.