Expending on my previous message I am actually astonished at the responses that the OP has got.
She has not done anything wrong: she is off sick, she has decided to hand out her notice and she has followed the correct procedure.
There is nothing incorrect about giving notice while on sick leave and in choosing to do it in writing as you are unable to speak to your line manager face to face.
She has been a good employee for 10 years and now wishes to leave. End of story.
The behaviour of the employer is appalling and it is truly bizarre that so many people think it is OK to treat her like this.
She should at least not have been locked out of her email account until HR had acknowledged her resignation and they discussed any outstanding pay and holiday pay and confirmed back to her the official date when her employment would end. If she had any uniform, door pass and so on, the employer should have also discussed with her how to return this. Standard procedures for any decent employers...
In some industries you do have to go on garden leave and immediately leave your workplace because of concerns about sensitive Information or clients, but you can be sure that the employee is made aware by HR of what is happening and communicated with them about notice, pay/annual leave and so on.
Seriously, what sort of places do some people work in?