i understand that you are caught in the middle with this one but i do see where your mam is coming from. She is taking alot on by shaperoningyou all to the party island and i know for one that i wouldnt be as brave as her lol. Im sure she is under no illusions that you will all be drinking and having a good time, and so you should but your friend must be pretty bad if she is saying no to her going and putting you under this pressure.
I would be honest with your friend and tell her if it was up to you then you would allow it but as your mam is doing you a favour by going with you all the final word falls with your mam. What exactly is it that your mam doesnt like about her? have you not thought about having a holiday over in this country perhaps as a compromise? ive heard cornwall is a great place for teenagers after exam time. Maybe if it was in this country she would allow your friend to go as if there was any trouble off her then her own parents coulkd easily come and sort her out.