I can't force this employee back to the office but I feel that we're denying her her rights.
What a strange thing to write.
You don't need to 'force' her back and how are you 'denying her rights'?
If I was in this situation, and I've managed similar, I would be sympathetic but clear.
I would outline her options, including ensuring she feels fit for work. If she indicates she might not be, she needs to go to her GP, and ultimately Occupational Health may need to assess her. I can imagine anyone might be in a poor state mentally with these events, so it's absolutely legitimate, but is her choice.
If she is fit to work, you would then need to outline any other short term practical support you can offer, with a clear deadline, before she returns to work. In my workplace (public sector, not UK), it would include force majeure leave, using annual leave, parental leave, and we would have some discretion over limited unpaid leave.
Working from home would not form part of this, as it won't solve her issue, and won't allow her work effectively.
It does sound as if she is angling for something. You still haven't explained if she had given her notice to return, which would have indicated practical arrangements were in place? It seems so strange that it would all fall apart so significantly so quickly.