If she's below average in year 9, she will (most likely) be below average in year 11. Which means she would be getting a mix of grade 3 (fail) and grade 4 (standard pass) in her GCSEs. Not great, not a disaster, somewhat limits her options post 16 as she will not be able to get onto certain courses and will have to resit if she fails to get a 4 in maths or English, but she'd most likely be able to find a pathway that suited her, even if it wasn't ideal or her first choice. That's as things stood before Covid.
Now you're proposing that she gives up on doing any schoolwork until she is back in school. That could be September, it could be later. When schools do go back it could be months before full time teaching resumes due to the need for social distancing. So she could be halfway through year ten before she's getting full time education again. She won't maintain the skills and knowledge she has in that time, so she will be going into the mid point of year ten with the ability she had at the end of year 8.
Those 3s and 4s will slide down to 2s and 3s. She will now have the barest minimum of post 16 options. She will only be able to resit the maths and English GCSEs at college so she will have a string of failed GCSEs attached to her for the rest of her life. She will be locked out of a lot of higher education courses, and a lot of jobs.
Why would you choose that future for your child?
You need to work harder. Contact the school, and ask for some help - but be clear with yourself that this is now part of your job, as her mother. This is a really tough bit, for both of you, it will feel like wading through quicksand sometimes, but your job as her parent is to get her through this, not to give up and let her sink because it is difficult.