Is it permissible for a man to marry a second wife and conceal that from his [first] wife, and is this considered good treatment, given his knowledge that concealing it will harm her more than his marriage [itself] would?

It is permissible for a man to marry a second wife without informing his first wife, because her consent is not a condition for the permissibility of polygyny; rather, the condition is justice among the wives. If a man acts justly and fulfills their rights, then this is precisely living together honorably. However, concealing the marriage without a preponderant benefit, or lying to the wife by denying it, is not prescribed unless it is for the improvement of the marital relationship, to avert a greater harm, or to achieve a more preponderant benefit. An exception to this is if the first wife stipulated in her marriage contract that he should not marry another woman except with her knowledge or permission.