First of all, how come the guy's mum reject the girl? Is the
girl too ugly for him?
And how come it's the guy's side who reject the girl and not the
other way around? How did the parents found out about the
relationship?
Anyway, it will also depends alot on the girl if she still wanna
have this relationship with the guy. It seems that the girl already
don't like your good friend anymore. Cos if she still likes him,
she would've suggested an alternative way to stay together. ->
Important point, she is ignoring your good friend and that's a
clear signal that she wants to end it on her own accord and for her
own good.
As his good friend, you should keep him busy with other activities
to help to get him over his grief. Nothing stays forever...people
will change. 
Good luck.
