Can the police enter a home with search warrant for someone else?
A warrant allows the police to search whatever location is identified in the warrant for the contraband or persons identified in the warrant--even if that means entering and searching person A's home with a warrant for person B.
A warrant is a court document which gives the police the authority or legal permission to enter and search a specific place for specific evidence or persons. The warrant must specify the location being searched. It must also specify what (or who) that location is being searched for (that is, for what or who are the police looking). A failure of the warrant to specify the location to be searched or the object of the search, or a mismatch between the warrant and the location entered (for example: the warrant specifies searching 35 Madison Street, but the police read the number backwards and entered 53 Madison Street) renders the search illegal and the results of the search inadmissible (or unusable) in court.
But those are the only restrictions on a warrant: that it must identify the location and it must identify what or who is being searched for. In particular, there is no restriction that a building or home or apartment must belong to (or be rented by, or otherwise occupied or controlled by) the person that the police are searching for (or who is believed to be the owner of the items for which they are searching), or that the police can only execute a warrant relating to a person in his or her own home. Rather, the police can enter any home (or office, or other location) in search of anyone--or anyone's contraband or evidence--as long as it’s spelled-out properly in the warrant.
Of course, the police first had to get the warrant, which means they had to have sufficient credible (believable) evidence to convince a judge that who or what they are looking for may be found where they want to look. There must be some connection between the person who is subject of the warrant and the location: it must be a friend’s house, a significant other’s or family member’s home, a place where he was at a party recently, etc. There must some reason why that person, or whatever items or belongings of his or hers which the police are searching for, would logically be there.
And there must be more than just logic, generally. There should also be some testimony or other evidence tying that person to that location, as, for example, a photograph of him or her at the location, the testimony of someone who saw him or her there, or property records showing it’s owned by his or her sibling or parent. Assuming that there is evidence showing some connection to the location, and that therefore, the warrant was authorized, then the police can enter Person A's home on a warrant relating to Person B.