Sourdough dough too runny after kneading and no rise during cooking.

Now you know what dough without salt feels like.  Can cut the salt in half but no saltt leads to all kinds of problems with dough handling and fermentation control.

I’d cut the dough water back to about 600g.  Sourdoughs naturally “loosen” as they ferment, which can make even a firm dough seem slack. 

Paul

Remember that salt or sodium is absolutely essential to human health.  Some salt will not harm your grandmother.  I don't know where you live, but in the US, what is commonly referred to as "SAD" or standard american diet, contains way more sodium than people need.  Most of this comes in the form of highly processed foods which use salt both as a preservative and as an attempt to make up for some of the taste that is lost when you don't use fresh ingredients.  Also some restaurants, even when preparing things with fresh ingredients, just add way too much salt because people used to eating the SAD have trained their taste buds to expect the salt.  If your grandmother is doing a good job of keeping her sodium intake under control by eating more fresh foods at home and avoiding or at least limiting those things that are unavoidably high in salt, then the amount of salt in your homemade baked goods should not be an issue.  I say this is based on the advice of several doctors over the years who have treated my grandmother, mother, aunt, and sister for high blood pressure, and my father and myself for heart failure, which also requires a low sodium diet.  However, this is not intended to overwrite any instructions she's been given by her doctor; I'm sure there are some circumstances where sodium intake must be much more strictly controlled, so if that applies to her, disregard what I said.

You might know this already, but baking is something like a complex chemistry experiment.  More so than most cooking, small changes can cause huge differences in your final product.  Unless you understand the chemistry and what the results of your changes will be, it's often best to stick very closely to the original recipe.  I used to watch my mother make changes to every recipe she made, but when I was learning to bake (mostly cookies and cakes), she made me follow the recipes to the letter, and I had to measure everything precisely.  She told me someday I would be able to make changes, too, but not until I had learned more.  In contrast, she allowed me to do other things, such as assembling a casserole for dinner, by eyeballing the ingredients, or adding extra of some ingredient (usually cheese in my case) that I wanted more of.

With the internet you can shortcut a lot of that hands on learning by simply googling.  You might want to look for low sodium bread recipes or try a question like "what happens if I don't add salt to bread?"  Just try to make sure you are taking your answers from reputable sites and/or check several places before you proceed.  Sometimes that's still not enough to get the correct information (as I am learning with my starter, which literally every site I checked said it would be ready to bake in 5-7 days.  ha!) but on average you'll come out way ahead of doing your own experimentation.

Here in the Netherlands there is a max percentage salt commercial bakers may add to bread. 
This is 1.5% and I have read that is also to minimal amount your dough technically need.  

So I never use less then 1.5%.
And that 1.5% of all the flour icl. the amount of flour from the starter/levain. 
Not adding it to the starter of course.