If I could give advice to someone on how best to take care of someone with diabetes, I would let them know to follow the golden rule. Treat others the way you want to be treated. I know that there are different requirements for taking care of a child with diabetes versus an adult. Having been diagnosed as an adult, this is where I'm speaking from.
Don't constantly remind them of complications. Do you want to be reminded every time you get the in the car of how driving without a seat belt could kill you? No? Aha. Complications don't always happen, and neither do car accidents.
Do join them in focusing on the positiveness of everything around them.
Don't give them the 'look' when they have a piece of cake for dessert. Do you want people questioning you on everything you eat? No? Aha. When eaten in moderation, and special occasions, cake is wonderful! Especially in the cup variety, with paper to pull off.
Do join them in healthy eating choices and enjoying that piece of cake from time to time.
Don't micromanage everything they do. Do you like to have your shoulder looked over when you're doing things? No? Aha. There is a learning curve, yes, but that doesn't mean we need to be watched like a hawk. Mistakes happen, and they help us grow.
Do ask if there is an area that you can specifically help with. Log books are a pain in the backside, and if I could have handed them off to someone to fill in when I tested I might actually have full.. and accurate... log books.
I believe that the best rule of thumb in taking care of anyone in any circumstance is to think of how you would want someone to treat you and do that.