Coming to this a bit late, sorry. DWP, how long has your mum been going to these meetings? I feel for you, as many years ago my mum went through similar. In her case, this lasted 2 or 3 years.
Some of the places my mum went to were spiritualist churches of the type Norks, Fattybumbum and Chegirl describe - ok, that was one thing: a service with healing, etc. added on. That's how I remember it, anyway. (I was about 11-13, and my mum often took me with her.)
But she also went to the tenner-in-a-village-hall type things, and these were very different experiences. Like MinnyMummy said, some mediums there were (I would say), well-intentioned but misguided; others were just charlatans.
Mum also used to have private meetings with a medium who was quite well known at the time (this was the early 70s). Till he got exposed in the national press as a complete fake. This was finally what made my mum turn away from the whole thing.
I know my mum did derive some comfort and reassurance from all of this. But, looking back, I also wonder if the 'emotional feeding' involved in some way prolonged the guilt and other negativity associated with her loss.
Is there any way (I'm sure you have been trying to think along these lines!) you could gently nudge your mother away from these meetings? I don't suggest you just say 'Mum, you're mad to think these people can help you.' Can your dad and/or your siblings/your mum's siblings have input here?
Sorry this is so long. And of course yanbu.
Wish you well.