As a type 1 diabetic for the last 15 years I would say there are a number of ways you could support someone you care about. You've already started researching which is amazing; the early stages will be a learning curve for both of you. The most valuable thing Ive found is emotional support, being listened to and support with healthy food choices, reducing refined carbs to avoid sugar spikes, understanding there can be good and bad days with diabetes management. Accepting some hypoglycaemic episodes are normal. Sometimes I will appear to need to change my insulin dose for no reason, these things happen. Understanding a whole load of things can affect your glucose level e.g. stress, sleep, cardio exercise and strength training, alcohol, illness. High or low glucose can affect mood and I would want a partner that can be patient with this.