I believe that marriage can be considered failed when you feel like you lost your ability to love the person you married.

Humans are not linear so saying things like: after you have kids marriage is over or true happiness only exists with kids make few sense to me, relationships are far more complex than that.

Women change after having kids? I would think that it would be strange if we did not, being a parent changes you - for better I hope- but that does not mean you are automatically neglecting your husband just because your kid is 3 ! What kind of absurd idea is that?

Long term relationships are possible:

http://www.psycinreallife.com/romanticrelationship/

they are just more demanding. Good luck for your friends and I hope they can find their way to happiness, whatever they do.