Yeah but that's not what you were discussing here. You stated Jill wasn't aware of MC doing stuff will Bella. And the surprise Jill had at going to Bella's house.
The only way that'll happen is if you as a player are actively perusing Bella and the only way that'll happen is if she likes you. So the making sure Bella isn't a big fan of you is a moot point in this case.
Now as was discussed yesterday, if you go to dinner with Jill and her friends, you're on the Jill path and theres no way of getting off it unless the MC chooses to. You can't swing back to DIK affinity and get rejected. And the dinner only happens if you choose fight for her, so the MC actively wants her here... So for him to do all this and then hook up with Bella, yeah that's all MC, some off Bella.
If you don't fight for her, you don't go to the dinner and she's not really an option anymore, despite the fact the game has pushed her such. Bella obviously likes her as you're on her path to end up on her dining room table...
So we've got a situation where Jill likes you, has told Bella, Bella likes you so has told her to go for it. Then Bella's fallen for you despite the fact her friend likes you. Again a shitty situation, betrayal maybe? But it's more niavity from Jill here as the MC had already called it quits by not fighting for her.