I understand your predicament, and it's hard to say no, especially to your mother. If you're pretty sure the treatment, whatever it may be, will be pretty useless but will please your mom and help her feel that she's not entirely helpless or worse, useless in all this, and that maybe she can make a difference for your ME/CFS, I would be inclined to do the kind thing.
But I'd also warn her that even fairly innocuous things, at least for other people, can have a devastating effect on your particular form of ME, setting you back badly and possibly causing extended harm, and that in future, it would be good if she'd run it by you before spending her hard-earned money on something that may be either useless, or potentially devastating. Then you can have a nice visit and talk it over, addressing the pros and cons, the possible risks and potential rewards ratio.
It's so hard for anyone, even mothers, to understand the depth and scope of this illness, and more than anyone else, they desperately want to help, to heal, to make things better for their child. If she's like my mama, she can prbably almost literally feel your pain while still having no idea about the nature or source of it. It's a hard place for her to be.
I wish I'd understood this better while I was going thru a different deadly disease, and was less than open to attempts mama made in my behalf. I'm almost crying, writing this to you.
So yes, do the kind thing. But be vigilant for side effects and reactions.
If worse comes to worse, and I dont think it will, you may have to stop taking/using it fairly abruptly if any signs of bad reactions pop up, and your mother will understand this better than a flat "No".