It's sweet that you're concerned about your friend Carl, but like other posters have said the 2yr engagement- I feel- shows they're not entirely rushing into anything, presumabley they are living together now, or, at least will be soon? That'll cement the relationship and be the touch stick of whether or not the marriage'll work, I would have thought. I don't know as people deal with grief differently, but maybe she has dealt with her mother's death in these last couple of years? Obviously the pain of losing a parent or anyone you love never truly fades away, but I wouldn't have thought that the marriage is some sort of knee-jerk reaction to that, other than making her want to keep hold of happiness and live her life to the full, i.e not let this man she's crazy about slip away from her. Besides, not being cynical, but if things don't work out they can always separate later on. Just be happy for her mate.
EDIT: Personally I don't see myself getting married for a long time (think my parents marriage/divorce has put me off for life!) I think a committed, loving, amazing relationship is all you need, not the actual document...but I know I'm heavily biased!