My School friend is engaged!! Watch
so it might work
SO YOUNG IMO.
I have two experiences of young marriage,
my grandparents got married at 21 and 24 and have now been happily married for 66 years.
my dad got married at 20 and it failed after 6 years, I haven't ever met her but I know that she ran off with someone else and he says that he wasn't mature enough at the time and got married too young. So it can go either way.
I personally have been with my boyfriend for 5 years (I'm 21) and we have been talking about marriage for about 3 years but want to wait til we can afford a really nice wedding since we only plan on doing it once.
I would say be supportive of your friend and try to understand that some people are ready for long term relationships much earlier than others.