I heard from a friend who gets bad migraines that you do not need to take a full dose, causing psychedelic effects, in order to get the relief/prevention. She said that a half dose prevented migraines for about two weeks. I do not know if the same strategy would work for cluster headaches, or even if it's a universally applicable characteristic of the drug, but if your mother is not the adventurous type or is unsure about tripping, she may want to try a smaller amount first to see if she can get the prevention without the mind-altering effects. Good luck in any case.
Yes it's true. That was a deciding factor to try it as she is against drugs for herself by fear of bad trip :)
The only downside (well except the breaking the law thing) is only that you have to be off the regular treatment to try it, so you may have to endure the pain for some days.