My mom has been diagnosed with arthritis at 43 years old and she is refusing to go to physical therapy is that bad for her?

??: Physical therapy dose not help everyone and it can be a good bit expensive ? What kind of arthritis there are many kinds you cannot make mom do what you think is right just like she probably couldn't make you do what you did not want to do / support mom the best you can and see what happens next.

Not always: As has been noted in other answers to your question, therapy might be helpful. Again depends on arthritis and where, and also what disability it is causing. For knee arthritis: therapy can improve motion of the joint, strength of the muscles surround and supporting the joint and balance. However, it will unlikely dramatically reduce the pain by itself.

It can be: Evidence shows that the most beneficial physical therapy modalities related to osteoarthritis are strengthening, aquatic therapy, and balance and perturbation therapies. They can reduce pain and improve function. The main thing is to find a type of exercise which is fun for her, so she'll actually do it.