Hey guys, some updates to this madness.
Karen recovered from her surgery reasonably quickly, and was out of our house mid-July. She left to go live with Mike, who is currently living with his parents (he just graduated school on a career path that leaves his future geography uncertain - no he's not a doctor).
Things were pretty quiet for a while after that. I know they've been sleeping in separate bedroom (his parents are pretty "old school"), which leaves plenty of opportunity for her to work her mischief. Weirdly, they are totally broke (I sent her $3 so her checking account wouldn't be overdrawn), and he's been working but she's been instead helping around the house with his parents (they're not old, and don't "need" the help) with no plans for work.
A couple weeks ago, Rob attempted to contact Mandy over Facebook. Karen had been ignoring him, and he wanted to be sure she was okay. Mandy responded, saying that she was fine, but did not offer details. Presumably, Rob still thought Karen had been living with us.
A few days ago, Karen texted Mandy imploring her to block Rob on Facebook, since she said he was coming to the US and might try to contact her (still under the story that Rob is "some crazy stalker guy"). She was very urgent, yet tried to put on an air of being casual. It was expertly done. It was weird. Mandy did not block him.
He contacted Mandy on Friday (via Facebook again). He was looking for answers. Apparently he had been planning to visit her in the US (still under the guise of living with us), and she bailed on him 12 hours before his flight was due to take off. She claimed she had schizophrenia (which is legitimately common in their family) and that he had to cancel the trip because she had to go to a mental hospital (this was of course, pure bullshit) and they couldn't be together anymore.
Mandy didn't want to answer (it's still her sister), so she had me do it. No way she was going to have this guy waste thousands of dollars on plane tickets too. Putting on my best "playing God" impression, I contacted him myself, and told him the answers he wanted. Oddly, the conversation wasn't that awkward. Naturally, he was crushed, very upset, and told me some pretty scary things. Apparently, he gave her $7,500 that was going to pay off her car (that didn't happen, Mike has been paying her car note), supported her the whole time she was there, and had just bought a house (350k loan!) for them for when she was allegedly going to move back in April and they were going to get married. He thought she had broken up with Mike a long time ago. I felt terrible for him; he seems like a reasonable normal guy who was dumb enough to fall into this. They had been "together" since 2014. He thought Mike was "the other guy," but him realizing that he was actually "the other guy" hit him pretty hard. His conclusion was that she's got some serious mental issues. I didn't disagree.
He thanked me, and requested that I tell Mike. Haven't done that yet. Not sure if I will or how I'd even do it.
Rob walking away is definitely the best outcome all around, even for Karen since she was trying to toss him under the rug anyway.