25 year age gap?

That depends on the people, but it's going to be hard to have an equal relationship when one person has almost thirty years of adult experience and one has four. He'll be more your dad than your significant other.

I know a couple where he is 56 and she is 27.

They have been together for 6 years and counting.

They seem quite happy..

An age gap of this sort is often discussed (mostly wrongly) about the odds for success. A relationship is always possible. This is not an election or a sports playoff game where predictions are helpful or particularly meaningful.

If it were my friend that asked me this question, I'd encourage them, if they' re happy and enjoy the company, to continue. Try to avoid other people's opinions like the plague because many people will express skepticism, cynicism, criticism...and frankly, jealousy.

One concern I've seen is that financial differences that often exist between 2 people of that age gap may affect the relative balance of power. Being supported financially can end up messing things up. Being in different stages of life as well as having social circles who are liking different social activities can by an issue too.

Lastly some of those life stage differences may be centered around the existing family or the desire to have or not have children. This would only matter if this was to turn into a long term relationship.

Isn't this the same situation you asked about here a few weeks ago? http://able2know.org/topic/312801-1#post-6137065