"We all know, in French culture, marriage don't necessary mean death to us part." Where exactly does until death do us part exist?
In the U.S. the divorce rate is around 50%. Truth of the matter no one is "stuck" with anyone. Women initiate 66% of divorce filings.
Everyone has their own "deal breakers". People who are unhappy generally don't stay. Marriage was never meant to be a prison. If someone is in a 12 year relationship it's because they want to be.
The reality is when it comes to love and relationships most of us (fail our way) to success. Very few people hit the relationship jackpot early on. If this were not the case we'd be married to our high school sweethearts!
Anyone who pursues relationships or marriage without taking the time to do some serious introspective thinking in order to figure out exactly what they want in a mate is prone to either choosing the "wrong mate" for them self or getting married for the "wrong reasons".
There are generally two reasons why a man doesn't propose.
1. Timing (He's happy being single or his focus is on other things.)
2. She's not "the one".
According to statistics in the U.S. by age 44 approximately 85% of all men have married at least once! I would imagine some the 15% who had not may be gay but now that marriage equality laws are passing around the country odds are we'll see even more men getting married!
However for the sake of argument if we assume the stats remain as is that means the odds are most men are going to get married.
Dating for 12 years may sound like a lot but one also has to factor in how old they were when they started dating. If they were age 17 then today they'd only be 29. Counting teen and pre-teen years while people are still evolving and maturing is not the same as two adults who have been living on their own with established careers and dating for 12 years.
It's also not unheard for men to end a long-term relationship of several years and becomes engaged to his new girlfriend of 6 months. Apparently they saw her as being "the one".
Based upon what you've stated your friend is not "in love" with his girlfriend and never was. He just "comfortable". Essentially he's robbing her of time she may have spent with a man who is actually "into" her. It's not the first time a planned "booty call" turned into a relationship of comfort.
However ultimately she also has a say in this. Maybe she doesn't want to marry him either! They both may be happy with the way things are! :)