Federal Rule of Civil Procedure 26
(b) Discovery Scope and Limits.
(1) Scope in General.
Unless otherwise limited by court order, the scope of discovery is as follows: Parties may obtain discovery regarding any nonprivileged matter that is relevant to any party's claim or defense — including the existence, description, nature, custody, condition, and location of any documents or other tangible things and the identity and location of persons who know of any discoverable matter. For good cause, the court may order discovery of any matter relevant to the subject matter involved in the action. Relevant information need not be admissible at the trial if the discovery appears reasonably calculated to lead to the discovery of admissible evidence. All discovery is subject to the limitations imposed by Rule 26(b)(2)(C).
To reiterate: relevant information need not be admissible at the trial if the discovery appears reasonably calculated to lead to the discovery of admissible evidence.
In other words: if it probable that searching your mom's/roommate's/girlfriend's laptop might lead to evidence in their copyright lawsuit, a plaintiff can go ahead and do so. Please keep this in mind. I always thought it was obvious that every computer or storage device in a household would be subject to search during a copyright infringement lawsuit. If this is surprising to anyone, I guess it is good that we have a story about it. IAAL, and this is not a controversial position. I wish it were otherwise, but FYI, there it is.