Really sorry to hear your wife is struggling so much. Where does she have the pains? Neuropathy is typically something which develops after quite a long period of poorly managed or undiagnosed diabetes, but obviously your wife has not had that as the cancer treatment triggered the diabetes and it sounds like her diabetes has been well managed since then. I am wondering if her pains are not associated with diabetes but perhaps another autoimmune condition. My sister developed Polymyalgia Rheumatica (PMR) about the same time as I developed Type 1 diabetes.... both are autoimmune conditions. She had pain everywhere and some days just breathing was painful. Diabetic neuropathy generally starts in the feet and progresses relatively slowly and will typically be tingling or burning or freezing sensation. I believe PMR tends to be more widespread, quite sudden onset and more generalised pain/aching. It is treated with steroids and my sister was also prescribed Methotrexate which is an immunosuppressant... ie the opposite of your wife's immunotherapy, which may have caused this problem too. I wonder if you need to push for a referral to a rheumatologist to get this possibility checked out.
Sadly it is all too easy for doctors to assume that whatever ailment you have is somehow connected to your diabetes and this is not always the case and in your wife's situation I think it unlikely that she is suffering diabetic neuropathy at this early stage, unless it was transient from reducing her levels, but clearly that should be resolving if her levels have been well controlled for 6 months.
It would certainly be worth doing some research on PMR and see if it fits before you spend a lot of money on the supplement
@Inka has recommended.
If it is definitely neuropathy then there are painkillers which can help which are Gabapentin and Pregabalin if the supplement doesn't work but I really think it needs more investigation if she is as much discomfort as you describe because I think it unlikely this is neuropathy, particularly if the pain is widespread and not just restricted to feet/legs and possibly hands.
I really hope you can find something to help her. Please push for a specialist referral. I think my sister gets blood tests which show inflammation markers. If it is PMR it can resolve over time with treatment although steroids will mess with your wife's diabetes management. My sister is now on a very low dose of steroids after 5 years and is gradually reducing it with a view to being treatment free eventually and unless she has a flare up, when she reduces the steroids too much, she is pretty much normal. I have another friend who had a very dramatic PMR event which landed him in hospital when he woke up one morning almost paralysed with pain and he is now back to normal and off medication, so there is hope that it can be resolved with the right treatment. I am not saying it is PMR but just that it is something to investigate and as an autoimmune condition it is another possible consequence of her cancer treatment.