RA is diagnosed in the UK by a consultant, a GP can do some mobility type checks and tests and take into account other symptoms, but due to the nature of it they don't diagnose albeit they may suggest I think you might have x. If she has managed to see a GP, be referred to and see a consultant and get the results back in such a short time frame then I would be really shocked. Not saying she's lying, I'm sure the GP did say she might have it or say it seems that it's caused by this, and although shouldn't judge as can't always tell what someone is going through, RA can be really debilitating and it feels like this is just her new ADHD to excuse being unorganised but this time to not being active. Again not wanting to minimise the condition or suggest everyone who has it should be a certain way as can't always tell and everyone is different, but going by her track record...
Realistically if she does or doesn't or might have losing weight will absolutely and categorically do wonders for her knees. As has been said she has the luxury of time, she can afford the gym or a swim she could go whenever herb is in childcare or in the evenings when Jim is back- it would be some 'me time' she harps on about needing as well. She does probably need some sort of therapy in regard to her eating as it's evidently disordered.