Like many of the substances used in Traditional Chinese Medicine (TCM) Rehmannia (aka Rehmannia 8 or Bawei Dihuang Wan) seems to be credited by it's proponents with being able to treat/cure a huge variety of ailments -
- Nephritis (nephrosclerosis, nephrolithiasis, nephrotuberculosis), pyelitis (nephroatrophy), albuminuria, and edema.
- Cystitis, cystolithiasis, cystotuberculosis, senile cystoatrophy, prostatomegaly, dysuria, urinary incontinence, and nocturia.
- Diabetes and urinary incontinence.
- Cerebral hemorrhage, arteriosclerosis, hypertension, and hypotension.
- Neurasthenia, amnesia, nocturnal emission, uncontrolled emission, impotence, and inappropriate erection of the penis.
- Lumbago, sciatica, deformed vertebra, numbness of the legs, and beriberi.
- Cataract, glaucoma, decrease of eyesight, and keratitis.
- Eczema, tinea, senile itching, vaginal itching, dry skin, and urticaria.
- Chronic gonorrhea, rectoptosis, climacteric disturbance and related disorders.
As you can see arthritis isn't on that list and it has to be said I've never seen treating arthritis as one of it's claimed uses.
There seems to have been some clinical trial work done on Rehmannia-based treatments for atypical dermatitis in dogs: Ferguson,et al and: Nagle et al but I haven't come across much else. And to be honest I'm highly skeptical that it's effective for much of the laundry list of what it's claimed to be able to treat let alone things that it doesn't!
The fact that the two trials (for AD) linked above happened and the dogs..well didn't die, suggests that it's not inherently dangerous for dogs to consume (assuming appropriate dosing) but there seems little chance of it treating your dog's arthritis.
As for how you can treat them; Assuming it's osteoarthritis (as opposed to something like hip dysplasia) it's basically the same as it is for humans - anti-inflammatory painkillers (specifically ones intended for dogs, don't go giving them ibuprofen as it's toxic for dogs!), diet, physiotherapy and low impact exercise - particularly swimming.
You (and your dog!) have my sympathies - osteoarthritis is a fecker! But it can be managed to reasonable degree and much quality of life can be had.