Looking objectively at the issue of a second Independence vote May can easily state that Sturgeons plan of requesting a timed referendum to enable Scots to effectively decide an UK/EU in/out is based on an entirely false premise of an early Scots accession to the EU, and as such is entirely without merit.
So should Scotland have a second referendum at all? Only about 30% (1.6mil of 5.3mil) of Scots voted for Independence last time and the SNP are not a clear majority Government in their Parliament so the argument for a clear mandate is on unsafe ground and as such Sturgeons argument that the SNP should lead the referendum and 'round' the question is similarly unsafe.
The UK Government is there for all its citizens and in the case of a potential second Scottish Referendum it is there to protect the interests of the 70% of Scots who voted no, or didn't or couldn't vote in the last referendum. The SNP, Salmond and Sturgeon have been proven to be unsafe with the truth and as such the UK Government is there to protect those Scots who don't believe in what the SNP puts out as their truth.
May should tell Sturgeon No, no referendum before Brexit is complete as your argument does not hold any water at all and is based on a deception of the Scottish public.
When Brexit is complete then the UK Government should say to the SNP, if you want another referendum then you should prepare a clear plan for a completely Independent Scotland outside the UK, and outside the EU. Then, and only then, should the plans be made clear to the Scottish electorate to ensure a clear-headed decision can be made with voters having full knowledge of what the future holds for an Independent Scotland.