I am in the UK but have some experience in management in our health service. I think the ?deliberate failure and the special relationship are red herrings in this case. I also think that Nancy's behaviour, while it needs addressing via the disciplianary process, needs to be divided out from the contractual issues she is citing.
If it was me who had the investigation of this I would be wanting to establish the following.
What specifically in Nancy's contract had been breached by chapter and verse?
How the change to the qualification requirements had been handled by the management. When I have done similar, there was a clear process in place whereby all staff were informed of
:The need for the change and the reasons for it
:The process and timescale for them to achieve the new required qualification
:The process that would be followed (training schedule, who would fund it, and so on)
:Any options for people who did not wish to take the new training
:The procedure for people who did not achieve the required qualification
I would want to see evidence that the process was fair to both staff and the establishment and that it had been applied equably over all the staff concerned, giving due allowance to any allowable particular needs of individuals.
From those two points, it should be easy to establish whether or not Nancy has a case either because there has been a contractual breach or because there has been a breach of the policies and procedures covering general terms of employment.
BONUS!! I would then be looking at OBJECTIVE and VERIFIABLE evidence of Nancy's behaviour towards co workers with a view to starting a disciplinary process, I would also be investigating her "resistance to change", how verifiable that was and what she had said and done.
GENERAL COMMENT It is of the first importance when handling staff issues to focus on what is objective and verifable. Use of gossip and innuendo is not going to get you anywhere and should be avoided. So far as ethical issues, any code of ethics which is cited or considered as a part of a disciplinary investigation should either be specifically established either within the T and C of employment or within the professional code of conduct of the person concerned. Realtionships among staff may not be explicitly banned but using that relationship to obtain preferential treatment, if provable, is probably covered under equal opportunities policies.
Keep it provable and objective.