Jill really liked the mc but since she put a huge emphasis on everything feeling perfect, if the mc is a DIK or didn't jump through all the correct hoops, while she decides she doesn't want to be his girlfriend, watching him fucking her best friend would still sting.
Logically she should be able to say, "It's OK, I knew Fuckface wasn't for me, but he's still a great guy, and Bella really needs to move on from her previous relationship, so this is actually a great resolution, it's a win-win-win for everyone!"
But emotions tend to screw logic up somewhat, her thoughts would be more like, "What!? I thought Fuckface liked me! And Bella knew I liked Fuckface! How could they do this to me! Even my sister's death is all about me! Me! Me! Me! Me! Oh my god! I will never forgive them!
"
They should have just closed the fucking curtains...
Of course if everything was peachy between Jill and the MC, and she was contemplating deflowering herself on his whack-a-mole pole, and then saw that shit going down, she's entitled to get a little annoyed.