She should make a will....
but you will later realise she has actively decided not to
so the in-fighting doesnt occur in her life time but afterwards
some hope.
and so you are sucked into what should have been settled by will
but hasnt been ( on purpose ) and I would leave them to it quite honestly
OK - nok is irrelevant
the rules of intestacy are will known in England
and they are here
https://www.gov.uk/after-a-death/overview
and you will see that any of the six can apply for letters of administration
and that the estate is divided by six equally
If the money [1ok] is in a jt account then the lucky joint holder gets it all.
If the money has been PUT into a joint account then half is the mum's and should be paid back - BUT he could successfully plead that the mum gave it to him as she wanted/intended him to have it
and finally any of the six can demand from the administrator who ever they be- estate accounts at the very end
Your fren' is trying to get you to weigh in on her side and I would resist all invitations, to be honest....