Had to chime in here on this one. After i finished college, I moved into my friend and his wives house. I knew them both for a long time and we all got along. They had just bought a huge house and weren't having children anytime soon. I viewed it as temporary, until I found a stable gig. I was working as an IT contractor at the time.
Anyhow, I got a front row seat on watching their marriage explode. Some questionable decision making on my buddies part (porn, finances, other women) had them at odds. The kicker was that he decided to buy a Lotus (55k) without telling her and hid it. As soon as he finally brought it home, it exploded. I moved out within 3 weeks.
I was by default, kind of in the middle of it. On one hand, I think my friend was finally being himself (not what other people thought he should be). He had gotten married right after college and had done status quo. I'm not sure he really was in love with her, but loved idea of marrying her and she was a good catch. However, she was kind of boring. The more I learned about what he was doing the more in good faith couldn't condone that behavior.
As it went on, I distanced myself from my buddy. Eventually, lots of thing came out into the open. I couldn't help feel bad for her knew she would be ok. After the divorce, I eventually talked to my buddy but things were never the same. Things worked out for the best.
Unless you are in the house, no outsider really knows what goes on in a marriage. So let them do their thing. Try not to takes sides if you can. Just be supportive and keep it at that. If you can't handle that then find new friends.