It's definitely not his fault, that marshmallow was Doomed the moment anyone started talking to Ritsuka. At least she does manage to put it out quickly.

"Well yeah. I know you wouldn't be able to do much while the sun was up, but it'd be a pity to miss out completely, right?"

And she thinks of him as a friend, so of course she'd be thinking of things like that. If she can keep track of the quirks of hundreds of Servants she can surely remember those of a handful of friends.

"Maybe, but... I kinda wanted to see if I could do it all nice..."

Look, the Servants who man the cafeteria are one thing, but Chaldea's current director is an amazing cook. It makes her kind of want to actually put in a little effort herself, sometimes!