Renter and landlor issue, help?

I have a situation with my landlord where in I breached our house rules and tenancy agreement by allowing a friend to stay at my apartment while I was away for work for a couple of day. The landlord informed me that he will take legal matters and will probably fine me to a maximum extend based on my contract and dutch law.
I had never caused any troubles for my 2,5 years of stay in his apartment and has always been a good tenant except for this instance. How do I proceed?