1. Fat is when you are in an air conditioned store and sweating bullets and panting like you just ran the Olympics because of your large mass. Fat is when you have to turn sideways and lift your rolls to fit through the aisle to get to the cashier to purchase your handfuls of candies you compulsively grabbed. Fat is when you can't wipe your ass (this actually happens). Fat is when the sweat between your rolls smells like mildew because you can't properly wash yourself and the sweat starts back up immediately after you've bathed. You are not fat.
2. While most people associate diet with some regimen, anything you eat is a diet. Just try to eat healthier. Change things slowly and permanently. For instance, avoid fast foods, candies, sodas, and foods with lots of saturated or hydrogenated fats.
The best way to lose weight with faster results is through exercise. Diets should be changed slowly and they don't affect your weight nearly as much if done properly. Exercise on a daily basis, or work up to it by doing something once a week, then twice a week, etc. Once you get into the habit of exercising, you'll be able to lose weight. When you reach your desired goal, don't stop exercising. This is a lifestyle change and should be permanent.
3. I wouldn't say it straight out. If they complained about their weight a lot and were obviously not happy with themselves, then I would suggest exercising more or doing something about it after a while. Initially, though, I'd encourage them to be happy with who they are.
4. Not directly to the point of being shocking or rude.
My personal experience:
Seriously, though. At 5'2" and 110 lbs, I can hardly believe that you would look fat. I used to be 110# (I'm the same height). I was definitely not fat. However, at 127#, I can definitely say I'm developing a layer, yet my friends tell me I'm far from fat and they wouldn't lie about something like that. While everyone else seems to like the way I look, I'm still putting in effort to get my body back to the way it used to look at 115# as I was much happier with my weight there. Below that and I've had people ask me if I was anorexic or bulimic (which is why I doubt you look fat). I would seriously put into question your SO's idea of beauty. If he likes stick thin like supermodels, then yeah, you may be fat in his eyes. (In which case, I'd refer him to this
link.) But from a level-headed perspective you sound like you're a far cry from fat. If *you* are unhappy, do something about it. If you feel fine about your weight, I'd suggest finding a new SO. =^-^=