Fifteen years ago, my husband, Peter, was diagnosed with type 1 diabetes.  It was the year after we were married, and it came out of nowhere.  The diagnosis hit us hard, as you can imagine, and seemed unbelievable because you just don't plan on these things happening to you.  We slogged our way through all the information we could find, learning all that we could about the disease and how to maintain the utmost level of good health.  With years of experience now, I feel I can offer some ideas on how to help support someone with diabetes (whether it's type 1 or type 2).

1. High and low blood glucose levels can cause mood swings. Knowing this, I try not to react when Peter's mood abruptly changes (even when it's aimed at me).  Instead, I see it as a sign that his blood sugar may be "off."  I empathize - he must feel awful having to deal with such involuntary highs and lows all the time.
2. Visit a nutritionist with a specialty in diabetes. We learned right away how to eat a balanced diet, what the properly sized portions are and, most importantly, how to count carbohydrates.  For those with type 2, the diet is critical and can be the primary treatment.  See Diabetesnet.com for recipes, carb counting, and information about the glycemic index.  The glycemic index measures how fast foods are likely to raise your blood sugar.
3. Don't resist change. We thought it would be impossible to make certain changes, such as not eating potatoes or pasta with every meal.  My husband could never imagine drinking Diet Coke - he consumed Coke Classic at a high rate.  But now, our "diabetic" diet is just how we eat and it feels completely normal.  I remember reading that it takes only 21 days to integrate a new routine into your life, to the point where it no longer feels unusual.  My point is, change can happen fairly easily and it will stop feeling like a sacrifice in fairly short order.
4. Think about healthy snacks to have around, and keep them handy. Here are some ideas from my kitchen:  veggies and blue cheese dip, fresh fruit, nuts, hard-boiled eggs, pickles, Greek yogurt, trail mix, sugar-free pudding.  The key is to have the food already cut and ready to eat, and to have it in plain sight, at eye level.  Otherwise, it seems the box of cookies is bound to be pillaged!
5. For type 1 people, keep glucose tablets (or your preferred fast-sugar item) available throughout the house and in the cars. We have them all over - in the kitchen, bedside, in the TV room, and travel sizes by the front door.
6. Exercise really does make a difference. Muscles use glucose for energy, and exercise improves the body's response to insulin in order to lower blood sugar.  It is hard to make someone exercise - and I don't like to nag - so lead by example.  Make it something to do and enjoy together.
7. Find the right doctor. Two things here:  One, there are several ways to treat diabetes; and two, the advancements in treatment options have moved at a fast pace.  So research the best endocrinologist(s) and get more than one opinion.
8. Check out the Continuous Glucose Monitor (CGM) that reads blood-glucose levels as they happen. A small beeper-size device shows the reading and the trend up or down.  An important feature is the alarm if one's glucose level goes out of normal range.  Thus providing the ability to react before the levels get too low or too high.  To learn more about the different CGM options, go to diabetesnet.com.
9. Find ways to reduce stress. Stress can cause blood sugar to rise (there is a good article on this topic at Medicinenet.com).  For Peter, he requires far less insulin per carbohydrate intake when he's on vacation than when he's working.  Whatever the reason, finding ways to reduce stress is a real benefit.
10. Don't assume you know how it feels to have diabetes. We can't possibly know how it feels.  The advice I follow is to listen and to learn.  Offering “advice” and telling someone how well off they are because diabetes can be “managed” is not always as helpful as it is intended.