Marriage is one of those things where only the people involved in the union have a meaningful opinion. You might have an opinion, I might have one, everybody on the planet might have an opinion, but the only opinons that really matter are the people who might get married.
Their families certainly have an opinion. That opinion needs to be considered, since it will directly affect the amount and type of support that the couple will get from their families, and that support is often crucial to making a marriage work (or not work). Even with that said, the opinion of the family still doesn't truly matter, it is the individuals that matter.
Advise your friend. Tell him honestly and openly what you think. DO NOT tell him what to do, that is for your friend and their potential spouse to decide.
Marriage isn't for everyone. If both (or in some cases all) of the parties aren't willing to give 100 percent to making it work, odds are very good that the marriage won't work. It isn't for anyone at all times, since they will have points in their life where they already have all they can handle, and taking on the burden of marriage is more than that person can manage at that point.
Marriage is a big step. It is a huge investment in terms of time, energy, and in many ways your soul itself... When it works, it is wonderful. When it doesn't work, it is tragic. Marriage is a huge investment, a huge risk, but if done correctly it offers the biggest payoff available to the human race!
-PatP