That was the situation under review by the Wisconsin Supreme Court in State v. Sobczak. The specific issue was whether the defendant's girlfriend of three months had authority to consent to the warrantless search of the defendant's computer. She had been using the computer, apparently with the defendant's consent, while she was staying the weekend at his parents' house. But when the defendant went off to work, she went clicking around, found videos of child pornography, and called the cops. I think they broke up.
The court found that the girlfriend did possess the necessary authority to consent to the search. Professor Orin Kerr thinks this is the right result:
[O]vernight guests have a “measure of control of the premises” when “the host is away.” Although this passage is hardly free from ambiguity — a “measure of control” isn’t clear about how much of a measure it confers — I would think that the most basic measure of “control” of a house is the ability to invite someone to enter the common areas of the home. If I’m right about that, Podella was exercising that right by allowing the officer to enter the common area of the home when Sobczak was away. Granted, she did so in one of those “unlikely” circumstances in which the guest found evidence of the homeowner’s crime, and the homeowner would not want the guest to invite in the cops. Olson indicates that if the homeowner is present and objects, the homeowner’s veto controls. But the homeowner was not present to object in this case, so I don’t think that conflict of interest matters.Chief Justice Abrahamson dissented. One of her disagreements with the majority was that it relied on the characterization of the houseguest as the defendant's "girlfriend," and the evidence didn't necessarily support the conclusion that they were so initimately involved. Instead, the record was that they met online, had been dating for three months, and that she'd accepted his invitation to spend the weekend at his parents' house while they were away. Then, in an apparent swipe at the majority, the Chief Justice says,
The majority apparently assumes that a 22-year-old man is having a romantic, intimate relationship with a 20-year-old woman whom he invites over for the weekend while his parents are away.What do you think, Mr. Gillette: did the majority make a reasonable inference?