Before 2016 it was legal regardless of reason why she has been taking it for 10 years. If her illness is real and that drug really helped to cure it, she might beg for lenience. However if there was never any disease, overlooked email won't be excuse. She knew she only needs it to help her playing tennis, not cure anything and missed email would be her fault for which she should be banned for maximum 4 years.
I think its clear to everyone that this drug was taken for performance enhancing reasons. The thing is, everyone is taking either legal or undetectable performance enhancers. Whichever way you cut this her 'crime' is the oversight when the rules changed, I don't see how using it as a legal performance enhancer would change my perception of that.
It changes a lot if she wants her 4 years ban reduced. Context matters. Imagine she really took it to cure herself for 10 years and then suddenly realized that your medicine isn't legal anymore. Of course this is her story, but if it was true she the tennis federation might reduce her ban, because all she never took it to enhance performance as such. If it isn't true, her crime is taking it in 2016.