Who is liable for the costs for replacing carpet?

My boyfriend recently moved out of a friends place where he was renting a room. While he was in the premises we had put a few small stains on the carpet (such as red cordial). When my boyfriend notified the person he was renting the room off that we had stained the carpet he was told not to worry as they would be replacing the carpet eventually. Now the people that my boyfriend used to live with are trying to make him pay for new carpet. We will happily offer to have the carpet professionally cleaned however we dont feel that we should be up for the costs of the new carpet as we feel it is the responsibility of the house owner. So who is liable for the cost for the new carpet?