Dear brother, remember, the we have no example apart from the Prophets and Prophet Muhammed s.a.w and peace be upon them all. Also remember that the only things which are forbidden are things which Allah or His rasool s.a.w have informed us are haram.
The Prophet s.a.w's wife was 15 years older then him, hence, a clear indication that it's permissible. Cultural norms are quite distinct from Religious sanctions.
All the best to your friend.