Tenant and I agree to terminate early

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I have a tenant that has been a bit of a nightmare... she was quite good initially, and then handed the property over to her Ex without telling me, cancelled her own Standing Order and since I have been unable to get hold of her. Her Ex has made one payment since to my account for the rent. Today I managed to speak with her and explained that she is in breach of her agreement by allowing him to live in the property. I also explained that she is still liable for everything and that I don't want him in the property.

I'm happy to put it down to experience and move on, so said to her that I would be happy to terminate the agreement with her with no penalties (assuming all is up to date on the rent front and that the property is in good condition) if she also wanted to do that. The other option is that she gets him out of the property and continues to be liable for the rent (not a good plan as she is now living elsewhere and has to pay rent there). She seemed happy with this suggestion as she now realises that she is still laible for everything. If I go ahead with this is there an agreement that we can sign to do this, or a process that we need to go through?

Any help is appreciated.

Thanks,
Dean