What is the ruling on a married man befriending single women, and what should a wife do if she finds her husband looking at dance and dating websites and accounts of single Muslim men, and viewing pictures of women, under the pretext of entertainment, while he assures her of his love?

You erred by spying on your husband and doubting him, which spoiled your relationship, because spying and bad assumptions are forbidden in Islam. And the husband must know the حرمة (prohibition) of illicit relationships and looking at forbidden things online, as these are forbidden friendships that lead to indecencies. You must help your husband abandon these forbidden acts and remind him that what he is doing is from the footsteps of Satan, and that he has many permissible options to enjoy marital life and fill his free time with what is beneficial. Get rid of your obsessions and worries if you see a response from him towards reform, and be ready to accept the legitimate options available as solutions to the problems.