I have just read the Bereavement Benefit page at DirectGov,and also dowloaded the Benefits Office Form.
I am sorry,but I am of the same opinion as yourself.That is,if your Mother and her Partner were not married(legally) then he is not eligible for the payment.
In fact on the Benefits Agency form it actually asks for proof of marriage,or civil partnership.
However,I do think it's worth asking your local Job Centre (or Citizens Advice Bureau) for help,ot may be there are other benfits that you Mother's partner can get in these circumstances?
I wish you (and him) wel.