I'm thinking:
Option 1: Theresa May's deal - she can move here any time up to Dec-20
Option 2: Remain - she can move whenever is best for her
Option 3: No deal - it would be easiest to come here before 29/3, but there is always the family reunification visa route, although that might come with restrictions on working. Also there's the issue of the min family income - her DH is grade 6 in the NHS, so I don't know if that is above or below the £££ limit.
Option 4: Almost any other deal with the EU (eg Norway + etc), then she can probably move here whenever
Option 5: A 2nd referendum. That would mean an extension to Article 50, and so she'd probably get another few months to make the move as it takes several months to organise a referendum (esp when the question is not even agreed!).
Maybe under option 3, she's doing the right thing, but even then it seems unlikely that option 3 will happen and even if it does, the new visa system would be so draconian that she would not be allowed to work.
So, on balance, she is highly likely to be able to take her time, wait to see what happens and decide once the UK knows where we are headed and her DH has had a bit longer to see if he likes being here??? But what if that is bad advice?!