A lot of people are addicted to sugar and processed foods. I don't know the exact foods you define as junk, but the problem with this type of addiction is, assuming you live in a western country, it's very normalized.
Another thing is it's nice to have raw ingredients and healthier options, but unless you plan on cooking it for her, it has to be convenient for her. Most pre-packaged junk food is convenient in that regard. She just tears it open, sticks her hand in it, and it's already prepared for her. You have to cook meat, you have to wash fruits. You should suggest to her healthier pre-packaged frozen dinners and/or snack options that are better than what she currently eats, rather than suggesting an overhaul of what she currently does.
Most people see multivitamins as pills, especially people who are a bit older. Logically, we understand they're not 'pills' in that regard, but they look like it, feel like it, smell like it, taste like it, and so unless she feels like she needs it she might not be inclined on taking random multivitamins (Or it might feel random to her that is).
I'm making a bunch of assumptions because I don't know what your case is. Besides her complaining, is there a specific issue she's struggling with? I get she's overweight, but does she deem that as an issue besides an aesthetic issue? She has to care for her to make change about it.