What is cause of unilateral buccal fat pad atrophy ? Have you ever seen such case?

The described effect is not related to Botox injection in the forehead and the eyes. It is purely coincidental. Perhaps see an ENT specialist for thorough examination. You can see pictures of the buccal musculature on my website below. 

It's not very likely that she has become resistant to Botox. It is more likely that it was not injected into the proper location. Botox parayses nerves that control muscles which then become inactive. However, if loss of fat pad was due, in some way, to Botox, one would expect it to resolve in the three to four month period of time. The injection of a corticosteroid can cause atrophy of fat. But this would have to be injected directly into the fat. The other consideration is that this may not be just fat atrophy. There is a localized form of scleroderma called "coup de sabre" which is unilateral and can cause indentation of all underlying tissue. I would suggest seeking a second opinion.              jlr