I cannot make a clinical diagnosis of your grandmothers pain over the internet, but it is important to exclude any other cause of pain in the arm that may be predisposed on the brachial plexus injury. A
benignBenign ear cyst or tumor
Benign positional vertigo static brachial plexus injury should not cause significant pain, unless the cause is neuralgic amyotrophy - an idiopathic neuritis that causes periodic pain and dysfunction of nerves in teh arm. Trauma could also result in a lot of pain, but I do not know what the cause of teh injury is.
So, perhaps reexamination or reimaging of her arm might reveal or exclude for example a fracture, progressiev neuritis, a mass , muscle spasm etc.
Other pain medications are available for neuropathic pain including Trileptal, Lamotragine, and a new one just on the market called Pregabalin. Other ways to reduce pain depend on the cause ie Botox for muscle spasm, Local nerve root injections for nerve root pain, and topical anesthetic like Capsaicin for superficial pain. For severe intractable pain, medications like methadone may be necessary.