@tutu10,
You don't.
She knows damned well that she's overweight. She feels it every single day in her knees (I know this from personal experience). She doesn't need you to tell her.
What you do need to do is make this a couples thing. You go out walking together every evening, for maybe 20 minutes. Surely you both have that kind of time to spare, yes? And it's not a race or a fitness walk. You stroll and you chat and you reconnect. You make it a pleasant experience that both of you want to duplicate.
For more exercise, you swim together if there's a place to do that. Ask her doctor for a referral to a physical therapist to help her with her knee. And yeah, I've got personal experience with that as well. PT has helped me tremendously, and I still do the exercises even though it's been coming up on 2 years since I last saw my physical therapist.
You also work on it from the food angle (
talk to your doctor first, please, as you are a diabetic). It's a helluva lot easier to say no once to an item in the grocery cart by putting it back than it is to say no multiple times when it's in the house. So put back the cookies and also the heavily salted and processed stuff. If she's been the only one doing the grocery shopping, then it's high time you pitched in and did it with her. Again, this is a pleasant experience. "Hey, let's get strawberries!" versus "Put back the donuts. How many times have I told you?" etc. you get the idea.
Shop almost exclusively the perimeter of the store: produce, meat, dairy, fish. Buy traditional oats (they can be fast-cook) for breakfast and ditch the sugary stuff. Make sure the bread you buy is 3-4 g of fiber per serving or more. "Lite" food is often terrible for you, because salt and/or sugar is added in order to mask that there's less fat. You can have full-fat foods so long as you don't have too much of them. Skim milk, though, is fine.
Don't go out to eat nearly as much. Restaurant food is larded up with sugar, fat, and salt. You can make much better foods at home. If neither of you are cooks or you don't have time to cook, buy a slow cooker and Google recipes. Don't use premixed slow cooker sauces; they're nearly always salty trash.
Instead, slice about three pounds of skinless chicken breast (trim and discard any visible fat), carrots (they can just be the baby carrots in a bag), celery, and chop onions. The vegetables should be around another pound. If your slow cooker's too full, cut back. If it's too empty, add vegetables. Full with water or with low sodium chicken or vegetable broth (that can be commercially prepared). Run for four hours and that's dinner for a few nights. Serve with salad and a couple of slices of toasted wheat (not white) or rye bread.
Substitute beef, etc if you prefer. For another recipe, soak beans overnight in water, discard the water and add to the slow cooker with low sodium chicken or vegetable broth, plus chopped onions and tomatoes. Run for four hours. If it's soupy, it's bean soup. If not, then it's filling for tacos.
Always have dinner with a big salad and always have that first. Drink water or skim milk instead of soda.
More cooking? Consider taking a class together, or following recipes. Even recipes that aren't reduced calorie, unless they're for desserts, are still going to be a lot better for you than restaurant food.
And, hang in there. I've lost 150+ pounds before, and this time around I'm at about 55+ off but have another 75 or so to go. My weight loss is maddeningly slow -- about 2 pounds per month -- but it's coming off. And I can attest personally to how much better my knees feel.
Oh, and a trip to her doctor is a must. Have her tested for thyroid function and have the doctor take a history to see if she's got PCOS (polycystic ovarian syndrome). Both of those make it a lot harder to lose weight but they are both treatable with medication.