Thank you very much for clarifying. A lot will depend on whether the manager was in any way responsible, whether directly or indirectly. She may not have been there when the incident happened, but was she in any way responsible for it, such as because the appropriate training was not given, she knew about issues with the carers that caused this and did not deal with them, etc.
These are just examples but you have to consider what links she may have had t the actual incident to bear some responsibility for it, even partial. If there are no such links then he can challenge the decision and raise a formal grievance about it.
In terms of her legal options, unless she is dismissed, her only option is to resign and then claim for constructive dismissal in the Employment Tribunal.
It is also worth mentioning that there is a possible alternative solution to this, which could avoid the need for legal action. That is where the employer is approached on a 'without prejudice' basis (i.e. off the record and with protection against these discussions being brought up in future legal proceedings) to try and discuss the possibility of leaving under a settlement agreement. This can be done by asking for a meeting, or it can be done in writing, via letter or email. Under a settlement agreement the employee gets compensated for leaving the company with no fuss and in return promises not to make any claims against the employer in the future. It is essentially a clean break, where both parties move on without the need for going to the Employment Tribunal. However, it is an entirely voluntary process and the employer does not have to participate in such negotiations or agree to anything. There is nothing to lose by approaching this subject with the employer and testing the waters on this possibility - the worst outcome is they say no, whereas if successful it can mean being allowed to leave in accordance with any pre-agreed terms, such as with compensation and an agreed reference.