I don't think that Google should be held legally responsible for what has happended in this situation. The counterfeiters, in this case, are the guilty ones.

It would, however, be in Google's best interest to be doing all they can to prevent this type of advertising in the future. Clearly, they need to do more to stop unauthorized parties from using a company's brand name for their keyword ad. They should also share with Montblanc any information they possess on the counterfeiters, if they have any legal ability to do so.