I'd say if you've made the commitment to marriage, you have in theory made a commitment to work out any issues in the relationship between you. So I think the guy should address the issues with his wife, maybe thru couples counselling etc. If it doesn't go anywhere, they should split. If not, and they're both happy living together in a non-intimate way, I think the guy getting involved with another girl is only ok if the wife knows. (not necessarily specifics, but she's consented to her husband getting his kicks elsewhere).

That's my take on it. Marriage is a commitment (prob why I'm not married) and should be taken seriously. The guy can always leave if he has problems.

If it's in a society where divorce and separation is not allowed, or very stigmatised, my answer might be different, but I assume your scenario is based in a Western country (US, Canada,UK, Australia NZ, European countries etc)