Sounds like he was really asking: How do I get my wife to change?"
Clearly his response and actions indicate he does not believe the problem is "with him". You might indulge him by asking him what he wishes was different about his wife,their marriage, and life.
This allows him to express whatever it is he feels (you) don't understand.
Ask him when was the last time he was "happy" with her and when did he feel like things started going down hill. After he expresses how he feels you might ask him if he believes his wife is happy/content with the way things are. If not, ask him what (he thinks) she would want or need to feel happy?
Should he answer that last one you probably should ask what is keeping him from giving her those things or doing whatever...etc
Note: This should be done in a casual conversational tone without assigning any blame.
Lastly you might ask him if he can see his life continuing this way until he dies or what he thinks will happen if things don't change.
People who are still "in love" seek ways to find compromises.
People who have "fallen out of love" are only interested in having it (their) way. Their mindset is "You & Me" not "Us & We". They're not a "team" anymore.
If there is any "good news" it's that he's hiding from their problems at home as opposed to running around in the streets with other women or looking for ways to avoid coming home. Another good sign is he wants to talk about it with someone. Most guys don't open up about issues in their relationships or marriages even with best friends. I've never had a friend ask me how he could improve his relationship with his wife. He must feel close to you!
Next time instead of jumping in with advice have him diagnose the problems from (his) point of view by asking questions. Everyone of us has our own internal requirements in order to feel loved, desired, and happy in relationships.
Get him to tell you why (he) is not happy with things (first).
Secondly you acknowledge his unhappiness without taking sides and then you get him to paint his "ideal scenario" of how he wishes things were. Afterwards you ask him what he feels is keeping things from moving in that direction. Essentially you're guiding him to his on resolution. At some point you have to ask if he's talked about this with her. If not why not?
Aside from that you might suggest they take a vacation just the two of them or have a weekend getaway to have some fun. Maybe have a double date.