Hi Kathy,
Rehmannia 8 will seldom create problems, and is fine to try first in this cat. The age is not a good fit, though, so I suspect there is more inflammation there than meets the eye.
The only problem you could see with BWDHW/Rehmannia 8 is a hydrating of any latent and unappreciated Phlegm in the airways, producing a cough that is worse in the mornings and on first movement. In that case, with the renal picture, I reach for San Ren Tang. Note that it is often helpful in low back problems, too, and that the tongue can be red in cats responding to SRT, making it a good fit. It would be unusual, however, for a cat on an exclusively raw diet to still be Damp. Was it a recent diet switch? Or does the cat also consume a lot of processed food? If so, SRT remains an option.
Lastly, a Kidney deficient cat with a red tongue can be chilly if there is a secondary Wind invasion due to Yin deficiency. The formula for that is Du Huo Ji Sheng Tang.
So those are three options. Likely one of them is right, and it may just be a matter of trying them out until you happen upon the one that yields the most benefits. Sometimes I use BWDHW and SRT together, so you could hedge your bets and start with that.
Hope the advice helps!
Steve
Steve