Warrants are necessary for a search in a criminal case unless an exception to the warrant requirement exists, and the remedy for not getting a warrant is any evidence obtained as a result of the search is not admissible at trial in a criminal case. In this case they likely could justify the search the under exigent circumstances exception. But, if your friend wasn't criminally charged then this does not apply and is not really a question for the criminal defense category. Generally for purposes of suing the police you would need to show they engaged in conduct so far outside of their authority as to be a gross abuse of it. Entering a home in search of a runaway likely would not qualify.
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