What is the cause of her severe arm pain?

This indeed, is a consequence of the psychic attacks underway. It is underpinned by a karmic history in which she did something to him to cause a grievous injury, and he swore revenge and is taking this out literally, on her right arm in the attempt to render it useless. This is a consequence of both psychic energy seeking to cause turmoil within the tissues of the arm itself, coupled with spirit meddler attacks being directed at that region of the body, and they are perturbing the nerves and this is the reason for the severe pain.