I am not convinced you have done anything wrong. Could be that the severely thick toe nail that is involuted has been pushed by foot wear being too tight on the end of her toes.
The nail does look like it is too long actually , considering it was done a week ago.
I have clients that I have regularly provided pedicures on for the last 32 years and as a natural evolution of some , have become thickened and involuted. They don't want to go anywhere else other than my salon, it is their choice. If I felt the job was out of my capacity , I then send them to my podiatry friend.
Maybe file them in future instead.
I clip holding the clippers at an angle and it nips away the nail, and then I file to neaten the cut. Occasionally I use sturdy cuticle nippers to nip at the nail instead.
I think it's her footwear that is the cause , has she gone back to the gym or changed her shoes recently? Is she on blood thinners? Only you know your client and can ask about lifestyle to help with an explanation.
Good wishes to you .