This strikes me as kind of a non-issues. It wasn't a meal someone had slaved hours over a hot stove, it was takeout pizza. If this person was on a diet and knew in advance that the meal was going to be pizza (instead of a typical holiday meal that includes meat and veg and salad), then I'm not sure why it's a big deal for her to have brought her own diet-friendly food. Grilled chicken to me isn't a "superior" food compared to pizza. If she brought over filet mignon or sea bass with organic baby greens, I could see where it would be weird. As someone who has struggled with their weight for years, I would tend to be sympathetic with someone who has made the commitment (so difficult!) to losing weight so much so that they plan their food in advance and bring it, even when it's a little awkward. Cut this person some slack.
I don't think I indicated that I thought she meant for her meal to be superior to ours.
Fair enough. I thought that's what you were implying when you said that you felt she should have brought enough to share. My mistake.
I'm not actually sure that I agree that the meal being pizza made it more OK for the guest to bring her own meal. Seems like a slippery slope to me.
The meal being pizza is only important insofar in that it's generally a difficult food to eat if you are on a diet. Yes, you can bank your WW points, but you'll still be starving an hour later if you only eat one slice. More "traditional" holiday meals contain enough healthy items to be serviceable to someone on a diet. (Or, at least they were how my grandmother cooked them - roast meats, steamed vegs, etc. You'd have to skip the mashed potatoes, but sometimes they were baked, etc.) If you were doing low-carb or low-fat, you could make it work. Pizza is really hard to make work on a diet.
I did another type of diet for awhile that would have made things difficult on two fronts. I did OA-HOW, which uses a meal plan from a nutritionist (for which pizza would have been nearly impossible), and I had a sponsor to whom I committed my food daily. This why, I had to know what I was eating for the day, and it took the guesswork out of meals (as well as that moment when you're wondering what to have for dinner and start snacking on crap while thinking about it.) This made it so I had to bring food to some family events and at times it was awkward. I had to eat 12oz of veggies (steamed or raw, or some combination), which is 3/4 of a pound. There's no way I could have brought enough steamed veg for everyone (it being not only kind of expensive but also a very large volume). Nor were they particularly interested in having steamed green beans on the side of their pizza or mac and cheese. But luckily, my family recognized that I was struggling to stick to a healthy way of eating, so they supported me and didn't make things difficult by judging my desire to avoid the foods they were eating. (I did not judge their desire to eat them. I wanted to eat them too!)
Regarding the salad dressing - some people are just picky about dressing because it's the only thing that allows them to choke down the salad they would rather not be eating. It's not a judgment about the dressings your mom has. I love salads, but I only love them with the right dressing. (Luckily I like a lot of dressings) But I was just abroad recently in a country where they only serve salad with a side of oil and vinegar - not my favorite. It made it much harder for me to choose the salads knowing I wouldn't enjoy it really.