Not at all there is a mandate to negotiate Brexit. If you're not prepared to walk away then you guarantee the most one sided deal so she is being mature. What realistic alternative is there? The irony is that if Cameron had been prepared to walk away and back the Leave Campaign he would probably got a much better deal so there'd be no Brexit now. I'm far from the only one I know who swung from Remain to Leave due to the failure to get any meaningful reform (confirming we'd never get it).

Either way there's an election due in 2020 a year after we Brexit so if the deal isn't acceptable we can elect a new government who can start new negotiations.