What is the ruling on working for a holding company that has usurious subsidiaries, knowing that the company in which the husband will work does not deal in usury, but there may be dealings between it and its usurious subsidiaries?

The fact that one of the company's branches deals with Riba (interest/usury) does not prohibit working in its other branches that do not deal with it. It is not forbidden for the husband to work, even if transactions occur between the two companies, as long as his work is not related to Riba or direct assistance with it.