Is my behavior correct in texting my friend's husband to help him solve his marital problems, knowing that my relationship with my husband and my friend is good, and the conversations have sometimes become casual?

Your speaking with the man is a grave error and must be stopped immediately, because it is a means to falling into the forbidden and a betrayal of your husband and your friend. You must repent sincerely and stay away from him, and beware of speaking to him about any matter. If the man’s purpose is legitimate, then he should speak to a man like himself.