Your question in full: My husband was getting out of his parked car and as he opened his door, a passing car in the opposite direction hit the car's door. There was a parked car across the street, so the passing car was trying to make it through the narrow space and barely nicked my husband's car door. My husband's car is fine, but the other car has a lot of damage to the side and side mirror. Who is at fault in this scenario?
State laws differ, but typically it would be your husband's fault in this scenario.
Normally, if you open your door into traffic, you are the one who has to take extra care to assure that a passing car won't hit your car door. Your husband then would be responsible to make certain he could open his door and get out safely.
If the passing car's driver was driving over his line or driving carelessly, then it's possible that both that car's driver and your husband may both be found partially at-fault for the incident.
Many states have comparative negligence laws. With comparative negligence, it's possible that both can be assigned some fault in the accident by the car insurance companies involved. In general, negligence is defined by auto insurance companies as failure to exercise the care required of a reasonable and prudent person in any given circumstance.
Even if there is negligence found in the other party's actions, it's likely your husband would still be found the one most at fault,and thus the one whose car insurance company would pay for the damage to the other car -- just for the percentage he was found at fault.
If your husband was found to be only 70 percent at fault, than 70 percent of the other party's damages would be paid under the property damage liability portion of your car insurance policy.
Luckily your car didn't sustain damage, so you wouldn't need to make a collision claim.
You can contact your insurance agent to notify them of the accident and discuss your state's negligence laws and how fault may be determined.