As far as I know, that item doesn't work for and isn't used for her, the vial labeled WG only works on the owner of Shadowed Curios as far as I know. If you want to make Diana fatter, the only way I'm aware of is to just keep playing the game and gaining weight until the lab opens up as an option to travel to (I can't remember the exact weight that triggers its introduction, but I think it was somewhere past 200 pounds IIRC), the character you interact with at the lab will be the one to give you what you need to help Diana gain weight.
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Yeah, since it's basically an extra/cheat given how much it costs, it'd certainly be nice to be able to use it on more than just the random NPC encounters. Thankfully there is at least the legendary sandwich late game that you can give to basically everyone in the game in order to make them permanently bigger, unlike the raygun only being temporary.
You only get 15 of the business cards for the maid cafe (though there are only 10 maids to find), if you run out of the cards (as seems to be your case) you can go talk to Gemini at the cafe and pay 100 dollars to get more. A tip for giving the cards to the right people, play with the sound on, and listen for bell ringing/chiming when you encounter and NPC while on the train or exploring the city, mall, and park, you'll also find one of them in Shadowed Curios. If you hear the bell, then that means they're someone who will accept the card and go work for the maid cafe, there's about 2-3 for each of the first three locations I listed IIRC, unfortunately whether their appearance will be triggered or not is pure RNG, so it might take a while, especially the ones on the train, if you get really unlucky.
Old post, so I don't know if you ever figured it out, but I'm pretty sure the different images titled "cheesecake" with the Shadowed Curios owner are for if you've gotten her to the different weight stages and ask for the cheesecake at that point; IE, the one simply called Gothcheesecake is if you ask for it before you've tricked her, 2 is after using the amulet on her, 3 is after using the weight gain potion, 4 is after the fairy uses her magic on Nicole. I haven't actually tested this hypothesis yet, but it seems to me that's what the difference cheescake images for Nicole are.