the most dangerous army you can face in any tournament is one that the player knows! tell your mate to simply take the army he's most comfortable with and knows best
if he's most comfortable with a huge swarm, than hopefully he'll do well with it!
the main problem i see him having is claiming objectives as 'nids simply want to eat everything in sight!
honestly though, i've seen many players try out a list they number crunch and figure what will kill most efficently, only to be slaped silly by an opponent with an average list, but who knows that list like the back of their hand!!!
cheers!