That said, not all yoga classes are created equal. I would say a more gentle practice may suit someone with arthritis better. Particularly Iyengar yoga has a philosophy of adapting yoga to help people with ailments. Yoga teachers aren't (usually!) doctors though, so I would encourage your girlfriend to talk to her physician about yoga. Or possibly Tai Chi. A good Tai Chi class will definitely help you develop a certain amount of strength (particularly in the legs).
As some added motivation, here are a few "ripped" Yogis. These guys aren't tanks, but they are good yoga practioners who have some muscle:
Some guy from New Zeland called Shane lol! (he's all over this site):
Now I'm not saying that these guys ONLY do yoga, but I'd say a lot of their strength comes from their yoga. I say this because to get to the level they are at requires a lot of practice (although I think Shane from yoga.nz does mention some strength training too).
"Let your food be your medicine, and your medicine be your food"