If the workplace has union representative, that's the first port of call.
If there's a whistleblowing mechanism in the firm, then that's a possibility because it's likely middle management making decisions that might not be in line with what the top line expects.
Next step would probably be your local Public Health department.
If push comes to shove, it's the Health and Safety Executive.
Tough to prove unless she's got an email trail though, it's all too easy for it to be brushed under the carpet as a misunderstanding.
Regardless of all that, if someone's been told to self isolate and their employer says feck it, it'd be wise for them to inform their employer by email or paper rather than just a conversation.