If the first spell of my opponent got defied by me, can I still use crumble sand to counter the second spell they played?

No. You cannot use Crumbling Sands to counter Spell B in this scenario.

The first spell your opponent played was countered by Defy, and a countered spell is not considered to have been played (Rule 425.1.b). FAQ #8877 further confirms that a Defied spell does not count toward conditions that check whether you've played a certain number of cards that turn. Since Crumbling Sands requires your opponent to have played another spell — besides the one you're targeting — and the first spell doesn't count, the opponent has only "played" Spell B this turn. That's one spell, not "another" spell, so Crumbling Sands' condition isn't satisfied and you can't play it to counter Spell B.