Generally speaking, your mother is not entitled to any benefits under current EU rules. Pensioners are free to live wherever they like in the EU but as “non-economically active persons”, which means in particular that they should have:
- comprehensive health insurance cover there
- sufficient income (from any source) to live without needing income support.
This government website provides a brief overview of the rules in the UK (see also Citizens Advice). The rules for people who are working or seeking work are more complex but
Those who are economically inactive, will not generally be entitled to income-related benefits.
(One exception would be permanent residents, e.g. people who spent at least 5 years working in the UK, but that does not apply to your mother.)
Since your mother's pension is under the level of the UK's basic income support, I suppose she might not qualify for the EU's right to free movement for non-economically active persons and would therefore fail the “right-to-reside” test for all means-tested benefits in the UK. This would not only mean that she is not eligible for housing benefits but also that she could even be expelled or asked to leave the country.