What is the ruling on a wife wearing trousers and short dresses at events without her husband's knowledge, when he disapproves of that? And what is your advice?

If the clothing violates the Shariah specifications that a woman's attire should meet when she is among other women, then it is not permissible to wear it, whether the husband forbids her or not, in obedience to Allah and His Messenger. The prohibition is further emphasized if the husband forbids it.

However, if the clothing does not violate the Shariah specifications and no harm is feared from it, it is preferable for the husband not to forbid her. If he insists on forbidding her, then her obedience to him in this matter is better and clears her conscience, especially since some scholars believe that a wife must obey her husband in everything that does not involve disobedience to Allah.

As for clothing worn in front of non-mahram men, it is not permissible to wear it in their presence, whether her husband forbids her or not, and the avoidance is further emphasized if the husband forbids it.