Well, after seeing the ballet I am not buying it, I suspect anybody can make their ankles more flexible just by getting educated on how to safely stretch and imitating what dancers do.
Here is a tool I found that some ballet dancers use that piqued my interest: The Ballet Foot Stretch from Spain.
From the Ballet Foot Stretch website:
BALLET FOOT STRETCH really works! As it lengthens, it strengthens, working on the superior part of the foot, at the same time giving you an extra "heel forward" support (cou de pied) as you stretch.I went to a couple ballet message boards to get a review and most the content is either neutral, positive, or simply question that foot work is cheaper. I looked for some You Tube videos of ballet ankle stretches but most I saw were more advanced than a swimmer with no dance training could accomplish.
[Cost €97.00 or $134.00 USA]