I do believe this is the first time Marigold has actually pissed me off. And I'm a bit torn, too. I can understand and empathize with her saying she's upset that Momo is suddenly excluding her - she wants to be part of the fun, and she has a hard time making friends. Momo suddenly has the ability to do things she's probably always dreamed of, and yes, that may mean excluding Marigold to some extent, but if Mari has so few friends that just ONE going to do something with someone else is devastating, maybe she needs to rethink how she interacts with people. Marigold sees it as "It's not fair, why does she suddenly get to have friends besides me" but as few friends as Mari has, does Momo have ANY besides Marigold? Momo has ALWAYS been there for Marigold and put up with a lot of her bullshit, including this little "not now" gem.
I think I'm with the guy who mentioned loaning money to a friend to buy a car. While giving something to someone doesn't obligate them to do anything for you, it's kind of rude to exclude them and then TELL them what you did without them, especially when you know they have trouble getting out but want to. Mari's jealous and sulky now, and to some extent it's understandable. She probably wishes there was some magic item that would suddenly make her popular. She feels like she bought that magic item for Momo and now Momo is ungrateful. (Not saying that's how it is, but maybe how Mari perceives it.)
At the same time, Mari's "not now" response makes me want to smack her. "You can only do stuff with me, when I want to do it" is essentially what she's saying, and that's just a nasty, controlling way to act. I wonder if she would treat a human that way, or does she just think it's not fair that a robot should be able to make friends more easily than she can...