As a whole the breed is more prone to dog and other animal aggression especially to smaller animals and small dogs. This is genetic, not through the fault of the owner- and was bred into them when they were fought against one another.
Many bull breed mixes do okay with other animals, many do not since they were bred down from animals genetically inclined to be animal aggressive. I do not trust them with other animals until I've gotten to know the dog. A true American Pit Bull Terrier will attack any animal near it, especially other dogs. They are bred to be what is called "game" and not release or stop even if they are suffering severe injury.
The likelihood of your friend having a proper APBT is low though and it is likely a bull breed mix. I'd exercise caution until you get to know the dog, but the dog might be just fine.
__________________ Kaira, Alli, and Douglas  |