After reading what you wrote, I have come to the conclusion that she really equates food with friendship and socializing.
When I was thinner years back, my best friend used to bring Dunkin Donuts munchkins and other junk food over. My other two best friends did the same. I was the one in the group that told them to please stop bringing it over my house, as I didn't want to gain any more weight- at the time I had about 25-30 to lose from my second pregnancy. I was successful maintaining this weight, until I had my tonsils out and another pregnancy after my tonsilectomy (sp). Of course, being tired and sick most of the time, I ate, and ate, and ate.....and here I am. Oh, and I also had a friend who owns a pizza and sub shop- you can imagine the feasts she would bring over, or have at her home for all of us to enjoy. Of course, she brought salad, too, for me.
To this day, my friends are all the same, much heavier than I am. I don't see them as much, as I live further away now. However, I did have a couple of other friends who told me they thought my weight gain was on their shoulders. I said no, it was mine- I really enjoyed their friendship, and at the cost of my health, continued to maintain it, and still do, but at a distance (I don't get to see them as much anymore, live about 2 hours away).
I am the one who should have put my foot down harder and made it more clearer like I did one time- I had my friend bring her boxes of goodies out to the car- and had her take them home with her. She got the hint, and didn't bring anything over for a long time, until an incident in her life happened that caused her to want junk food all the time (her husband was sick, and eventually died from cancer). I didn't have the heart to berate her at that time. However, I told her I wasn't going to eat them, as I wanted to stay healthy and lose weight. Of course, I didn't succeed, but that's on my shoulders, not hers.
What I am saying is, some people really do equate food with socializing. Most of us oldies were brought up that if you visited a family member or friend's house, you brought something to eat with you as a courtesy. Get togethers with family and friends always have food.....birthday parties, retirement parties, weddings, holidays, think about it, it's always about food, and in some cases, drinks (alcohol). So, many times, people like your co-worker don't always realize that they are doing this- it's ingrained in their minds to share their food and friendship. We all know how deadly that can be, but at the same time, these friends can also be a God send to us, too, in other ways. I wouldn't have traded them back then for anything, and I feel that way now. Maybe just saw less of them for my health's sake- we got together daily, sometimes from early in the morning til later in the evenings. We had a lot of great times, too, and when we weren't eating, we were laughing and having the times of our lives, while the kids all played, LOL
I hope your co-worker can eventually see the harm she is causing. Again, it may not be intentional- this may be something she is just so used to doing, she isn't aware of it. Bringing food to your office might be her way of just wanting to say hi, but she uses food as an excuse to drop by. She sounds very lonely to me as well- so food is her vehicle to conversations. Maybe a little one to one chat about this may do her and you the world of good.