Interestingly, sway backs rarely cause the horse pain when they are genetic (read that somewhere a long time ago). The issue here is the saddle fit.. how the heck are you going to get a saddle on this horse that fits? Her coupling is quite strong (note the slight roach ahead of her peak of croup).
Mostly this horse needs feed and muscling.. and helping her will require a lot of trotting up hills on a loose rein.. rider in a two point.. to help her build abdominal muscles to support her back as much as possible.
She looks more than 8 years old to me.. and she also looks like the spinal processes on her withers are adding to her height.
She has a nice hind leg and her front leg is a bit tied in at the knee. If you can get a saddle built to fit her, as hard as it is to believe.. she may be OK.