A great takeout DBL by West. Given that the suit West has doubled is hearts, West has absolutely guaranteed a 4-card spade suit. In a perfect world West would also have 4 diamonds & 4 clubs. But the bridge world is rarely perfect.
East (Advancer) is normally expected to bid their longest suit (in this case diamonds). But given that East has reason to believe that a golden fit in spades is guaranteed, I would expect East to opt for spades. Since East has at least 10 HCPs, they should also jump. East should bid 4S which West should Pass.
Ain't bridge a great game?