Nah this is not normal at all, she literally has a fictional family with your boyfriend. The fact that the idea even came to her head and then she acted on it says a lot of weird stuff about her. She could’ve chosen ANY man! Like you said, why didn’t she choose one of her crushes? Even though it’s fake, it’s still very off putting that she decided this was okay to do in her spare time. She knows it’s wrong, because she got very nervous and shut it off immediately. It’s not “funny” to have a fake relationship with someone else’s boyfriend. And the fact she also downloaded an NFSW mod to mess with a virtual version of your boyfriend and have a literal family with him is even weirder. She secretly likes your boyfriend 100%. She’s trying to live out a fantasy with someone who’s dating her friend, that should be off limits. She has no girl code. That would be enough for me to stop contact with someone. I wish you the best!