Depression. What can be done to help.

Dear doctor, I actually don't know how I could explain his problem he is married with one kid. He had a love marriage with lot of opposing from both their family. Life went on smoothly without any issue for several years. Now his wife is working abroad for last 2 years. He says that now his wife have least interest in him mainly on sex where as he wants to have it. Later he observed her love towards his younger brother. His brother is outward from family. Don't speak with any members from his family he only speaks with her. His marriage is fixed with the girl he love. My friend and his wife had many fights with this relationship. Once his wife told him that she loves his brother more than him. He started tracing her mobile he saw many messages between then which seems like more than what is needed between a sister and brother. He is total depressed he told his wife lets consult a consular She told no need as she don't want to break relation with both of them. What can be done to help.