The more sudden the onset of the disability, the more likely the person will have trouble coping. The big thing is the loss of independence. Having to give up driving, or depending on someone else to read things to you can be very devastating. It really depends on the individual. When they develop a vision impairment, some people isolate themselves, losing confidence in their ability to get around.
Support groups can be tremendously helpful.
Be patient and non-judgmental. It may take the person longer to do something, but let them do it. Don't rush them or grab things from them. The more sudden the onset of the disability, the more likely the person will have trouble coping. Don't assume what the person needsask what will be helpful.