Compared to shooting someone or breaking their arm, I'd say Aikido is non-violent. We are giving a person a /chance/ to not get seriously hurt, despite them attacking us. I'd say if someone has hit the point in Aikido where they could drop a street attacker, they have put some real time on the mat.
Depends on what that someone already knew. The op already has training in boxing and karate.
He may be able to incorporate the Aikido fairly quickly.