If I see one of our people manifestly unfit to drive, I WOULD physically take the keys from them to prevent them from driving but that would be an absolute last resort based on my judgement call that this person is about to hurt themselves and that's the only way I could think of at the time to stop them from hurting themselves and others. There are other strategies that are available:
Physically position yourself between the person of interest and the car door and either call or have someone call the cops or the top management. As long as your butt is resting against the car door's key slot, the person can't open the door. If the person of interest is already in the car, try to reach inside and cover the ignition slot with your hand. Do not use more force than necessary to prevent the person from driving away. If you are too late and/or the person of interest is too strong for you and the person of interest is driving away, call the cops.
Given that the management is sponsoring the holiday party and that the party is an official company function, the management of the company may have a duty to intervene with the person of interest given the employer-employee relationship involved, which falls under the doctrine of special relationship. The management has an incentive to intervene because having a clearly inebriated employee drive off a company function won't look good for the company in a court of law. Have one of the top management instruct the person not to drive and have the person understand that the top management will call the cops if the person tries to get into their car. Have the top management offer to drive the person home.
Given that the party took place in a restaurant, the restaurant's management would have a clear incentive to intervene i.e. the possible loss of its liquor license plus legal liability, if a clearly inebriated patron were driving off the premises. Have the management of the restaurant intervene and prevent the person of interest from driving home. The restaurant can have one of their employees offer to drive the person of interest home or call a cab on behalf of the person of interest.
Your friend should have asked a couple of others to back him up. The person of interest would have less likely to be in denial if she sees a consensus around her that she is in no shape to drive.
The person of interest's judgement was clearly impaired by her condition. Your friend should not have merely taken her "no" for an answer and should have called for others to back him up. Having the boyfriend get involved is not a bad idea but two pre-conditions must be met: 1. He is actually present at the party - and there is no indication that he is; and 2. He himself is not drunk as a loon.
I expect that there will be several answers but I think they will all be based on the premise that she can't be allowed to get into her car and that physical contact with her to prevent her from harming herself and others is an absolute last resort to be applied when no other alternatives are available.
Your friend's mistake was in trying to resolve the situation alone, by himself. He was operating blind, in total ignorance of the options available to him.