Well, I think that it's sad that her family were critical, without offering advice.
I think that 18 is a bit young to be thinking of marriage, but it can work out.
I would suggest, if I were her mother, that she holds off as far as an official engagement is concerned, for a while longer ~ maybe until 21, because even that is young ~ and see if she feels the same way then.
You know what they say ~ marry in haste, repent at leisure. Your friend is young, there is plenty of time to settle down a little later.
However, if she is determined to marry, then the best everyone can do is to give her support and make her big day special and memorable ~ in the best possible way.
"The secret of success is constancy to purpose" - Benjamin Disraeli.