Sure, why not? If your friend is suspected of dealing drugs, or possession of illegal drugs - he/she will be raided on a regular basis, with just cause, until they get caught - then charges will be laid. If your friend is innocent, then there's nothing to worry about. If you've got nothing to hide - hide nothing!

Absolutely. Your house can get raided twice in one day (or more) if the police establish probable cause. Your friend has the right to ask for and receive a copy of the search warrant that was undoubtedly signed by a judge and gives the police the right to enter the residence in question.