Does she like fruit smoothies? You can blend up some of her favorite fruit, yogurt, some milk and a little honey. Some of my mom's favorite fruits for smoothie are fresh pineapple, banana, strawberries, blueberries and cantaloupe. Put it in a glass with a straw and let her have at it. She loves fruit but has a hard time chewing it. The smoothies work for her.
Once she's in the habit of liking the fruit smoothies, you can then start sneaking in some veggies with it, things like beets, carrots, spinach, etc.
Also, please read and compare labels for the processed things you buy for her. Most of them are just carbs and empty calories and not very nutritious. Rather than Boost, look into something called AdvantEdge from Abbott Labs (They also make a good variety of high protein meal replacement bars.) Atkins is an alternative brand for the high protein drinks and meal replacement bars but I think the AdvantEdge brand tastes a lot better. Walgreens and probably most drug stores will have these items. They're also available on Amazon as a subscription so they get delivered to the house on a regular basis. It is also a lot cheaper from Amazon.
Rather than the Activia, look for some good quality Greek yogurt such as the Chobani brand. It comes in a wide variety of fruit flavors, has full fat, lowfat and nonfat versions, is a lot higher in protein, lower in calories and sodium, and they don't use chemical thickeners, but strain the yogurt to achieve thickness.
My mom also has dementia and she takes a good daily multivitamin/mineral supplement. Because of that, I can focus on getting as much protein into her as I can with high protein drinks, high protein meal replacement bars and her favorite snack foods. She tends to eat a lot of sweets and the various chocolate meal replacement bars seem to satisfy her sweet tooth while getting some extra protein in her. She has a difficult time chewing and loves the Blue Belle chocolate and vanilla ice cream I buy in 4 ounce cups (they're just the right quantity for her), as well as easy to chew crackers. She also likes to have oatmeal in the morning with a banana sliced into it.