Yamada's Boss: "Three of our part-timers quit this month... Oh dear... I wonder why?"
Also Yamada's Boss: "Oh... I better call Yamada first thing in the morning to get her to come in on her day off, even though she explicitly told me that she had something planned for that day, and even after she already agreed to cover some of the day shifts for the part-timers who quit, despite the poor girl seeming overworked and tired..."
Hopefully Yamada's Boss within the next 3 chapters: "What do you mean that Yamada quit? Oh dear... I wonder why?"
In all seriousness though, I do hope Yamada refuse the request. Even if it's only an hour, there's still the time it'll take to get there from her location. Kase will start running at 10am, judging from the time Yamada woke up, it's probably close to 9 am. So if Yamada watches the store for an hour, she won't be there when Kase starts running, and by the time Yamada finishes up watching the store, she'll have to rush over to Kase, and depending on how long it'll take for Yamada to get there, Kase will probably be done by then, even if Yamada skips getting fresh clothes on and stuff.
So yeah, Yamada is better off just politely declining, hanging up, spending the time getting ready and going over to the event and cheering Kase on... But knowing how nice Yamada is... I'm pretty sure she'll end up going to the store to help out and then pushing herself to get to Kase and maybe even collapsing.