I have had one case that is similar, on synthyroid for years. She said the best phase she had was the time she was on selenium, zinc (after my advice)+ a little iodine.
Unfortunately she stopped after half a year or so;as she was well, and taking all that supplements was boring.
She regret this now, as the last year has been a never ending illness of different kind.
I would advice your client to participate our forum, and decide wether or not she will go for the full protocol.
Diabetes; the ones I know that are on a low carb high fat diet (butter, coconut oil) has managed to significantly lower their insulin use. This diet also make them loose weight.
Chromium and other trace elements are needed; she is probably very low in several minerals / elements.