your mother is probably worried about you. even though it might not be your friends fault your mother may feel that she needs to protect you from people that could hurt you, such as friends. if this is upsetting you badly, next time your mother says it just tell the truth. you'll feel so much better.
Mother's always have someone to blame and they'll always be one of your friends that she won't like. She probably just doesn't like the look or personality of this particular friend and is not very trusting. Talk to her about it, I'm sure she'll understand if you just tell her.