I can chime in on this, as this is my condition. I have excessive lumbar lordosis curve, sometimes called a swayback. My trouble with kettlebell swings became apparent when I leveled up in weight and developed a sharp pain near my sacrum, just above my pelvis. These pains stopped when I stopped swinging, but returned within ten minutes of swinging the kettlebell again. I tried troubleshooting on my own but was unsuccessful. At the time the pains developed, I did not have an SFG coach in my area, so would have to travel 5 hours drive to the nearest SFG. However, last month a new SFG coach appeared near my town, so I booked an appointment with her to have my form assessed.
Coach Gisele identified that, because of my excessive lumbar curve, what feels like "vertical" to me is actually leaning backwards a bit, to "11:00." She had me "lean forward", stopping where I felt like "1:00" - which put my back to true vertical. This is not something I would ever have identified on my own, because as with many people with excessive lordosis, my upper back compensates with increased kyphosis curve, so I have a very bendy back and it isn't obvious where 'vertical' is. But it has done the trick. I feel like Gorilla Grodd, as I do feel like I'm leaning forward, but I'm now swinging 12 kg without back pain.
Coach Gisele also identified some issues with my Turkish Get-Ups, which were also contributing to my back pain and were also influenced by my excessive lordosis. Turns out I need narrower starting positions for my limbs than the traditional '45 degrees', in order to access my best strength. If I start in the traditional described positions, it places too much strain on my spine to pull me upright from windmill to lunge, but with the new positions prescribed by Coach Gisele, I can raise up easily and without pain. I offer my experience not as suggestions for your client, but as insights as to what else might be going on with her back. As with me, the solutions may not be obvious.
(nb Coach Gisele is awesome, if anyone in southern Alberta is looking for an SFG coach, I definitely endorse her.)