Plan your play : Opning lead is Q♥

Comment
Considering the points East holds in the other three suites, he makes the fair guess that East should hold some strength in hearts with at least an Ace or a King. His 6NT bid is therefore a reasonable bet.

West's bidding
As a 2nd bid West must not bid 1NT, for he is unbalanced. West must not bid diamonds again, for he holds values in two other suits.
The 2♣ rebid is the proper choice.