Can a lanlord gain access to a property without your permission?

Hi all... my girlfriends landlord was made aware tat because she had started a new job and obviously was required to work a week in hand his rent would be a week late....she did notify him of this and has never had any other rent arrears with him...however he has called today threating her saying he will gain access to the property tonight either by her letting him in or by him kicking the door in??? he has also said he will send his "friends round to collect the money??? is this legal??