Friend got my oyster card taken off him..?

Basically, i lent my friend my oyster card (bus card), i know it was stupid but we thought there would be no inspectors on a sunday.. there was. so he got my oyster card taken off of him and got a �25 fine, and i have to pay �10 to get another oyster card.. my mum has to ring up and get it for me.. but she is going to kill me! what shall i say to her?