My partner started a new job 4 weeks ago, gets paid weekly and hasn't yet paid any tax, her boss has told her today that this is because she ticked the wrong box on her P60 and that she'll get hammered for tax this week and won't receive much in the way of wages.........is this how it works, is it legal?
Yes. If they don't have a P45 from her last employer [for the current tax year] they are obliged to apply an emergency tax code until they have a response from HMRC to a P46 application for tax code.
This means she'll pay tax without any personal allowances until then, but will get back any overpayment in the next wage that has the correct tax code applied.
Yes, she has been underpaying tax so it's a good idea to put some or all of the 'windfall' aside for when the tax system catches up. They'll only recoup what should have been collected earlier