Icg76 is right, please get a solicitor of your own, legal aid will be allowed, and that way you will have a meeting with them before the interview to clarify things, also they will meet you on the day a bit beforehand, which can be reassuring, no comment for sure, anything said can be used, again as Icg76 has said. My own son was in a similar situation when he was 15, we attended the station and as soon as they saw the solicitor with us the wind in their sails flattened somewhat, dont do their job for them, my son, me and the solicitor all went in and my son no commented each question, and we left, a few weeks later a letter arrived, no further action. In my sons case it was lads messing about, and the one who started it came off worse and spitefully reported it as assault, tbh i think the police knew this and had better things to concern themselves about but had to go through the procedure. Try not to worry but get a solicitor. Oh and the boys friend as a witness? i am sure more than one witness is required for any court, but i may be wrong...Good luck to your grandson.