@OP -- You can start by buying some healthy food? No more processed foods, no more butter, NOTHING. Just straight up Salad & Lean/Red meat. Somewhat encourage her to drink a lot of ice cold water (tell her you read it from a china study) -- it'll increase her metabolism, so it'll help in the process. Seriously, it's all in the food you eat too. I don't know the right word for this -- but maybe change the aura of the house? You know, let's say you have a bar inside your house ... Renovate that bitch and replace it with a treadmill. Replace your E! magazines with Gym stuffs ... Simple things like that will bring a change (Unless she's dead inside and have no motivation to change whatsoever).
Now that's over with. Don't tell her you bought all this shit because you want her to lose weight. Tell her you wanna start eating healthy, YOLO bitch!
Don't invite her to your program, let her taste the stuffs you're eating instead, & don't ask if she likes it, just feed that girl. lol
Only really try to vocalize yourself of trying to help her if she wants it, if she tells you that she wants to lose weight -- then invite her to your program.
Hope I helped