Fattening others

As you considered letting her take her time? You can give her any shake, and simply have her drink it at her pleasure. Give her a reasonable time limit to drink it all. It's a bit slower than making her hungrier, but she might prefer it.

Well tbh most shakes are gonna lead to fullness lol but there are obv levels to it, cake shakes, heavy cream, oils, will all make it quite thick. My personal favorite is a nighttime half and half or heavy cream blended with Ben n jerries, but that may be too thick, although it’s. Maybe just start with half and half and a little bit of ice cream or peanut butter, slowly start adding in heavy cream and more ice cream?

if she really dislikes being full that much then maybe you need to appraoch this differently. rather than trying to make her push past her dislike of feeling full, find ways to help her consume more calories in a comfortable sized serving. find high calorie foods she really enjoys. make a shake that can be kept in the fridge for 2-3 days that she can have half a cup of at regular intervals throughout the day. and encourage her to have regular snacks so that the focus shifts from you trying to get her to push past feeling full to instead just doing your best to make sure she's never empty.

this will probably make for slower progress, but the last thing you want to do is push her too hard to get to that full feeling she hates and damage her relationship with food as a result. if she does want this it should be something she gets to really enjoy too, so find ways to work within her limits and comfort zone for now.

if it's legal where you are then thc might be worth trying, if she's okay with that of course

If she doesn't want to feel full but does want to gain I'd say maltodextrin is your best friend. It's what bodybuilders use to bulk bit if you're not exercising you will gain weight in fat and you can buy it cheap on the internet. If she doesn't like to feel full but does want to do shakes I'd recommend throwing a spoonful in the blender along with some nondairy milk and ice cream (which feel less heavy in the stomach than regular milk and ice cream) along with a flavoring of her choice (fruit juice, chocolate syrup, etc.)

Nix this idea if she has a gluten allergy though as a lot of the malto on the market is derived from wheat.

I would be careful tho, maltodextrin is synthetically made and usually used as a food addictive. It's basically sugar, except even stronger and more simple to digest, which means it can quite easily lead to diabetes