I don't really need to repeat my comments on Sky broadband, there are a few threads by me on this forum about my experience/problems/"oh god please help me" to give a general idea about how I've found the service. All other comments would just be against forum rules to the extreme. All I can say about it is new ISP + lots of new customers signing up = disaster. Look at the Talk Talk fiasco when their free broadband service started. Some Sky broadband packages aren't free, but they're cheap and it attracts a lot of people. Now I think they're having problems sharing the bandwidth out among everyone which causes connection problems. Maybe in a year or two it will improve, but for now you should consider an ISP that's been going for a while. Also, you could follow surfmonkey's advice and phone up a few ISPs and see if they can give you a free speed test on your line so you get an idea of what speed to expect.
Ask Sam click here and see what he says. If BT Wholesale's ADSL Max is not available at your friends exchange then he's stuck with 512 unless one of the LLU service providers services are avaiable, Sky, Be, Bulldog, Talk Talk etc.
If ADSL Max is available and the estimated speed is given as 512, take it with a pinch of salt. The BT checker which provides this information is very unreliable and it's possible that he could get much higher speeds than those estimated.