What is your advice to a wife who lives with her mother-in-law, who has abandoned the house and food for no apparent reason, and refuses to discuss the matter despite her son's attempts?

Our advice is for you to look into the cause of what happened, apologize to her, and ask for her forgiveness if you made a mistake against her. Treat her well and honor her, as that is part of honoring your husband and having a good relationship with him. If she wrongs you, then be patient with her.