Final answer:
The nurse could be sued for libel, as it involves making written false and damaging statements about another person, and the written notes in question could be viewed as such if they are untrue and harmful.
Step-by-step explanation:
The nurse in the given scenario could potentially be sued for libel. Libel is a written form of defamation that involves making false and damaging statements about another person. In this case, the nurse's written notes suggesting illegal drug use by a client, if found to be untrue and harmful to the client's reputation, could constitute libel. Unlike slander, which pertains to spoken defamation, libel refers to written or published false statements.
Therefore, if the allegations are not based on fact and have not been substantiated, the nurse may be at risk of a libel lawsuit for having written such notes, especially as they have been seen by the client's daughter. The other options, malpractice and negligence, typically involve a breach of professional duty or standard of care which is not the primary issue at hand with respect to the nurse's notes.