It sounds like your friend has really bad luck or perhaps is doing something wrong.
I supose we could ban all powerful dogs.
Don't ban them. Neuter them. Soon, the breed would be gone.
Here locally we just had our 3rd Pit attack of the year. Three went over a 6 foot fence and attacked a meter man at the house next door. It took two people with metal pipes to get them off of him.
So you tell me. I have owned and known people who owned dogs all my life, and I have never seen another breed that was such a timebomb. Are owners to blame sure. But I have owned Goldens, Labs and Springers and none of them have ever attacked anyone. When my Springer gets annoyed by my grand nephew and neice, she just looks at me mournfully and goes in the next room and stays there until everybody goes home. The same was true with my other dogs when these kids parents were kids.
My Lab and Springer were intense natural hunters and water dogs, which based on their breeding, you would expect. The Golden, god bless him, was just a big sweety that didn't hunt anything but his food bowl.
Any dog can be trained, but all dogs have instinct and breeding and Pits were bred for years to attack and keep attacking until either their opponent or they themselves were dead. Someday that instinct may be bred out of them much like the Golden's hunting instinct, but that is a long way off. And until then they are going to continue to be a hazard in most situations.