Trust me, just 'cause it's a thoroughbred it doesn't mean their bad news. In this case I agree with Kristy. Ignore your friend. I ride a thoroughbred named Wolli and he is very gentle and very sweet. Not all thoroughbreds are go go go. Some can be very calm and sweet. Depending on which one though. Some of them have been on the track but haven't been fast enough to win races, so they get sold for sometimes hacking. They might get very fast because it's in their blood and if they've just come off the track but after a while if you do some methods and keep them slow they will eventually calm down. One of the other reasons if they've come off the track and are fast is because they've been taught to do that.
I think you should just do what you think is right for you. If you want to keep riding him continue. But if you don't...
Then that's up to you, not your friend.