dry rotten fabric
This link may help. At the end of the post, she explains how she replaced the fabric:
Hello. This can be a frustrating situation. We have had the same circumstances due to the outdoor UV exposure.
Some manufacturers do you supply replacement slings. That might be worth looking into. In fact we have a chair set from the 1980s and we replace the sling for that pair chairs. It might be best to check with your manufacturer for a perfect fit and solution.
Otherwise DIY might be an idea to consider.
Here's a video that will take you through the steps. I hope this helps. Wishing you the best.
Here is some ideas I found for you, Aynot. Also a couple places to find this fabric.
Home Depot and Lowes sell the slings and there are numerous places online you can order them. I would suggest going to an outdoor furniture store to be able to see the fabrics and decide what you want. Good luck to you!
Here's another option for you: https://www.youtube.com/watch?v=B7ph2PP-tPg
You can paint the fabric to give it more strength
You would have to replace the fabric.
Outdoor chairs that have “failed fabric” should have their foam looked-at quite seriously, before fixing the fabric.
This is because the foam breaks down as well.
I did this with rotten canvas with my swing. You could do similar and get new pillows: https://happydealhappyday.com/fix-broken-canvas-porch-swing/
You could purchase replacement “slings” for your chairs online:
We replaced them. It's not an easy job and it's not cheap, but cheaper than buying a new set of chairs. Here's the DIY if you want to peek! https://www.exquisitelyunremarkable.com/2017/07/how-to-replace-patio-furniture-slings.html
Once that fabric wears through, it needs to be replaced, cannot be repaired.