Is a husband's taking his female colleague in his work car and her sitting next to him considered an unlawful seclusion (khalwa), and does the wife have the right to object to that?

It is not permissible for a woman to ride alone in a car with a non-mahram man. However, if there is a need for her to ride with him, and there is no fear of temptation (fitna) and no suspicion, some scholars permit it, provided that she does not sit next to him and does not converse with him. Therefore, the scenario mentioned in the question is impermissible, and you have the right to object to your husband, as this falls under the category of enjoining what is right and forbidding what is wrong.