Insurance is a good gauge of driver/vehicle/age related incidents. It's all based on risk and stats. Sadly the sensible well behaved young drivers fall into the high risk idiot braket that the majority create. Hence they all pay.

I'm 20 and have a 330i manual, I was 19 when I bought it. Had a Skoda Fabia before that. 8 months later I'm still here and the car is too. Best advice has been said already. Make sure his mates aren't tossers wanting to see him drift it. In the dry he's gonna be hard pushed to do anything silly, and it being an auto he should be safer still. In the wet if you are stupid with the DSC off you can lose it pretty quick. He will try and turn it off, he's 20 your not going to stop him. I I always drive mine with the DSC partly off, it has two modes, one push of the button puts it in Dynamic mode, and a longer hold turns the system fully off. I prefer it in Dynamic because it allows a little bit of slip which you can react to, and failing that it will catch it for you. I have found before in the wet with the DSC fully on if you react quickly to a slide you end up compensating for it and so does the car meaning you over compensate, but that will all depend on the driver.

No idea how experienced he is, but it can be difficult to predict the car in the wet with the DSC off. Sometimes you just go round the corner pretty quick just spinning an inside wheel, no drama, and other times you can have both rear wheels light up and find yourself at full opposite lock pretty quick. I know I have found myself running out of lock before, so I know that I haven't got enough talent for that sort of driving. Also as mentioned rear tyres aren't cheap.

Just respect the car, and the laws of physics and he will be fine.

The throttle goes both ways....

FWIW when I was 18 I regularity drove a E320 Merc, Auto. Still here.