The opponents were playing some version of Acol, weak NT.
She didn't know what to do with a balanced 19 count over the opening bid, so bid 2NT thinking that showed a stronger hand than overcalling 1NT (I have since explained how to show a hand like this after RHO opens, and how a lot of people play a jump to 2NT).
They ended up letting it through. Apparently on the last five tricks, South cashed her last diamond stop and then played a club to declarer's ace, allowing him to take the remaining tricks. Aside from that, the question is if you are defending 3NT with this hand, how would you go about planning the defence, knowing partner is marked with a bust and you will be leading out of your strength frequently?