Rule 1.9 conflict
A conflict involving a former client under ABA Model Rule 1.9.
Rule 1.9 governs duties owed to former clients. A lawyer who has formerly represented a client cannot represent a new client in the same or a substantially related matter against the former client without the former client's informed consent in writing.
The "substantial relationship" test asks whether the matters are sufficiently related that the lawyer's representation of the new client risks the use of confidential information obtained from the former client.