You've had some good advice here about the position regarding the flat and I'm glad she is getting support.
Looking at the other aspect, I think it is very likely that your friend has been unfairly dismissed.
The basic requirements for a fair dismissal are:
- Issues should be dealt with promptly
- The employer must act consistently
- The employer must investigate to establish the facts
- The employer must inform the employee of the problem
- The employee must be given an opportunity to put their case before any decision is made
- The employee has the right to be accompanied at any formal disciplinary meeting
- The employee has the right to appeal against any formal decision
Unless all of these steps were followed your friend has been unfairly dismissed. From what you've posted here, I suspect she has at the very least been denied any right of appeal. I also wonder if she was given a proper opportunity to put her case and was informed of her right to be accompanied to the disciplinary meeting (if there was one).
If she takes her employer to tribunal for unfair dismissal, her compensation will be increased if they have not followed all the steps above.
Even if they had followed all of these steps, I doubt an employment tribunal would agree with that lending the keys to her flat to someone else was a disciplinary offence unless it posed a clear risk to the security of the pub or there was something about this in the company's policies and procedures.
I suggest she contacts the HR department as soon as possible and tells them what is going on. From what you've said, I think it is quite likely that the landlord may be acting without the knowledge of the chain who employ him. I suspect they would be very interested to hear about his behaviour.