The problem here is a lack of depth perception. With one eye, it’s difficult to tell how far away something is whether that’s an incoming knife or the edge of a building. This could be very dangerous for them while just doing parkour really rely on being able to judge how far away something is. Even just a few centimeters could be the difference between a broken ankle, a broken arm, a broken leg or worse from a fall. There’s a good chance that when they lost their eye, they really lost the ability to do parkour the way they used to.
Parkour is not a fighting style. So, for an untrained fighter having lost an eye that could be really bad if the way they keep their skills sharp is through street fights. A fighter who trains regularly can adapt to it and may even be able to fight just as well as they used to in the long run, though their fighting style will have significantly changed in order to adapt to their injury. However, they’ll need time, a good work ethic, a solid instructor, and understanding training partners to help them get there.
Because of how fast knife fighting is and how quickly it ends, I can’t really answer that question. The lack of depth perception could lead to more mistakes and, when fighting with a knife, all you need is one to end your life. If they can’t tell how far away someone is then a miss could be fatal much in the same way it would be with parkour.
That’s all I’ve got.