Being someone who operates in (currently) EASA land, and who was diagnosed as being a Type 2 diabetic (Diabetes Mellitus) - now in remission - the current 'gold standard' test that is used to ascertain status in the UK, is the HbA1c test (glycated haemoglobin). The threshold limits as stand currently (unless corrected) are that less than 42 mmol/mol are non-diabetic levels, between 42 and 47 mmol/mol are Pre-Diabetic levels, and 48+ are Diabetic levels. The HbA1c test is an average of you blood glycated haemoglobin over approximately the last 2-3 month period, as this is generally the accepted timescale over which blood haemoglobin is completely replaced within the human body. The effects of statins and blood glucose effects are well known, and yes can prove detrimental in increasing the HbA1c figure over time, but equally this may prove more beneficial in some individuals as opposed to the increase risk of elevated blood glucose and associated effects (neuropathy, retinopathy, CVD etc.). A high sugar diet in the previous 10-days generally speaking will not make a huge difference to nay HbA1c test conducted during consumption, or immediately after - there is a school of thought that suggests the last 4-weeks of moderated glucose (carbohydrate) intake is moderately beneficial to reduce HbA1c figures.
I would suggest that your friend invest perhaps in a home HbA1c testing kit, or maybe day-to-day self monitoring using readily available testing kits as sold in most high street chemists? yes, there is a cost of course in doing these - but if nothing else, will assuage the fears he/she may have and help to monitor what is really happening. If you buy the right kit, there are some great apps and bluetooth transmitting functions that make report production and trend monitoring very easy. And in the early days, dietary methods alone seem to have good results in regaining normalised control. Its worth (ney, essential) to consult a dietician or GP before embarking on a diet method as there are effects that can alter lipid profiles and alike. Be wary of the suggestion of going straight to the medication route (metformin etc.), as there can be some 'explosive' results (think bowels!) in individuals, and adds more restrictions in medical renewals.
The UK CAA guidelines to renewals and re-gaining class 1 status, are all available on their website and well worth referring to - along with your AME of course!, as they ultimately have to sign you fit again once controlled levels are achieved and no other issues apparent.
Hope this is all of use.....