Assuming you work for a private sector employer:
No, the supervisor does not have an obligation to let the employee know the meeting may lead to discipline. The employee has the right to request union representation, but asking management IF you do or do not need a union rep present is not the same as asking FOR representation. The employer can truthfully answer "No, you don't need a union rep present" because in fact if you don't ask for one, you do not need to have one there. Even stating that you THINK you need a union rep present may not be enough to trigger the employer's obligation.
If a meeting starts out as a routine work discussion but then changes to one where there is the possibility of discipline, the employee can ask for his or her union representative to be present.
If the supervisor continues the interview and uses the information for discipline after the employee requests union representation, the discipline is not supposed to be used against the employee. This doesn't always happen, however, because the courts have not consistently enforced Weingarten rights. Hopefully your union has negotiated something into the contract that protects these rights, such as that management is required to remind employees of their right to union representation; if an employee requests union representation, the investigatory discussion must stop until the union rep is present; and no information learned between the time an employee asks for union representation and the time the union rep arrives can be used to support discipline.
Be prepared for some employers to claim the employee did not request representation, even if the employee did.
Note the right to representation only applies to INVESTIGATIONS. If management has already decided on discipline and is meeting with the employee to announce its decision, no Weingarten rights apply.
It is best to state at the beginning of the interview "If this interview could lead to my being disciplined or terminated, or if it could affect my working conditions, I request my union steward (or representative, or officer) be present. Until my union steward (or representative, or officer) gets here, I choose not to participate in this discussion." Many unions distribute wallet cards to their members with this kind of information.
if the employer has in the past ensured union representation for this kind of interview even if the employee does not request representation, it may have established a past practice. Usually, a practice must be consistently applied over a long period of time before a past practice is established. Your union should know the answer to this question. (If you are asking on behalf of the union, I encourage your union to consult with the District or International office, and/or to retain an attorney on its own, even if just for the occasional question or representation.)
If you work for a public sector employer, then state law applies, not federal law. Most state labor relations statutes follow the federal law, but there may be some differences, so you must check with an attorney licensed in your state.
twitter.com/MikaSpencer *** All legal actions have time limits, called statutes of limitation. If you miss the deadline for filing your claim, you will lose the opportunity to pursue your case. Please consult with an experienced employment attorney as soon as possible to better preserve your rights. *** Marilynn Mika Spencer provides information on Avvo as a service to the public, primarily when general information may be of assistance. Avvo is not an appropriate forum for an in-depth response or a detailed analysis. These comments are for information only and should not be considered legal advice. Legal advice must pertain to specific, detailed facts. No attorney-client relationship is created based on this information exchange. *** Marilynn Mika Spencer is licensed to practice law before all state and federal courts in California, and can appear before administrative agencies throughout the country. She is eligible to represent clients in other states on a pro hac vice basis. ***