not necessarily.
On the weight thing, some carbon forks can be HEAPS lighter than 531. The average 531 fork (one that's not too ancient) is about 700g, and some all-carbon forks are as low as 300g!! That is a lot for just one component.
What fork is he looking at buying?
The wrong fork could make the bike ride worse, but the right fork may improve handling. I put a cheapish Roselli carbon fork (from John Kennedy) on one of my 531 bikes, and it now handles better, but I tried some other forks that were too stiff.
Generally, I would say that most carbon forks would be less springy than a 531 fork, but they corner better. The carbon forks I have don't have the same bounce and spring as my old steel forks.