Can your landlord let the police in your apartment if they do not have a warrant?

Question Details:

My fiancée was in his and his brother's apartment along with myself. His brother was not home so we did not answer the door for the police. After about a hour of knocking the landlord let them in even though his brother was not around. They arrested my fiancée because of domestic violence against his ex wife. All that happened is they got into an argument and she called the police and said he hit her. Is any of this illegal? Important Notice