How many calories are you looking to get in? Is this person very picky? Do you know what kind of flavors they like?
Any dairy issues? Whey is a good start for this if they are OK with it. Hemp protein would add fiber. Figure out what carbs they like, add those, and add some good fats.
The artificial ingredient lists in the pre-made meal replacements are really bad, I wouldn't personally do those or have an elderly person do them.
What about one of those pre-made fruit smoothies with some added protein?