You need to contact both original suppliers, let them know you are acting for MIL, and tell them that she does not wish to change supplier, whatever one company may say to the other.
Get them to confirm, in writing, that they have been instructed to remain as the desired supplier for whichever utility they are providing...if they are approached by any other supplier regarding changing, they are to get written confirmation from you before any actions and decisions are taken...
I spent a year and a half working for SSE when the domestic energy markets were deregulated...to think that these practices are still occurring is disgusting...to switch supplier WRITTEN and SIGNED consent is required...all a verbal agreement does is confirm that they can send a form for you to sign requesting the change...it does not confirm the change itself...
Whilst you are at it, send a complaint to
OFGEM, outlining your concerns at the treatment of a frail and vulnerable member of society...