If Kerrie is appointed as Executor of your late partner’s will, she is classed as the Personal Representative of the Deceased and she able to deal with your late partners assets as though she is your late partner. That would include removing any messages which were in the Deceased account.
Legally, there is nothing you can do sorry to have to tell you. You can report it to Facebook but in all honesty I think they are unlikely to do anything for the reasons I stated above.
I’m so sorry that I could not give you a more favourable answer.
Can I clarify anything for you?
Please rate the service positive. It’s an important part of the process by which experts get paid.
We can still exchange emails.