She will need records from her drs, so getting those records an making sure they are correct as to facts of her condition..
Often things are left out or even typos, errors..those need to be corrected..
Much depends on if she has been getting ongoing medical care for the condition/symptoms..If not, it's hard to prove...

The sticky threads are the best place look for tips & help, suggestions.. Some may be older posts, but they can give you ideas to web search and check for updated info..