They do not have to have the warrant WITH them, so a key question will be when it was issued. You can help him by hiring a criminal defense attorney to go over the search line by line, word by word. Tip may be enough for a warrant, but hard to see a basis to just bust in. See an attorney.
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They should have had a warrant, unless there were additional circumstances you have not mentioned. An attorney may be able to suppress the evidence found, which would likely mean the charges being dropped. Tell your nephew to remain silent. He should not discuss this matter with ANYONE, including you, and especially law enforcement, until he speak with an attorney. He should go meet with a few attorneys and hire the one he feels most comfortable with. Most offer free consultations.
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As the other posters have stated, to conduct a search the police need a warrant or there must be circumstances present permitting the police to conduct a search under a warrant exception. Based on the facts you stated it appears a warrant was provided, although it was not presented when the search was initially conducted. As the others have suggested, your nephew should seek immediate counsel from a criminal defense attorney to determine if the search was in fact legal.
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