Where was the recording taken? In public, there is often no expectation of privacy, and you might be fine recording something that takes place in public.
In private circumstances, such as on a telephone call, the rules change quite a bit.
As for whether the threat is a basis for a UD action, that may or may not be possible. She threatened you, not her landlord. Also, it isn't clear what type of agreement your Ex has with your mom as to the terms of her rental.
The above is not intended to be legal advice, but may be used for general information. Please contact an attorney for specific help tailored to your needs. www.figgardenlaw.com