Being friends with someone and dating them are very different experiences.
Abusive and deeply manipulative partners are often able to hide that side of themselves when hanging out with friends, only to go home and focus their dangerous side on their partner.
In many cases, abusive partners take extra precautions to appear good in front of friends and family, this double persona helps intimidate victims from coming forward, since they're less likely to believed.
However, in cases where a friend or family member shows abusive traits outside the home, it's statistically safe to say that they're far worse in private. If you discover a friend is abusive, it's important to cut off that friendship and help their partner get out as soon as possible. If you think they're open to change - seeking counseling and professional intervention for the abuser is also an option.