Need help!

My mum has been signed off work since November. She is due to meet with her HR team on Tuesday as she is being absence managed. She is on SSP. Today she received a letter following her Work Capability Assessment, telling her she had limited capability (lCWRA). We are so pleased as she is in so much pain. Can she resign from her job now? She hates it and they are being awful to her. Job centre appeared to advise there wouldn't be any delays or sanctions as a result? She doesn't opt in to an Occupational pension. Also now she has received this how long will it be after today before she receives this? Will it take three months? And do you know how much it is