She doesn't have to be, she can file dual-status if she wants. But she needs to fill in the resident T1. If she ticks the box on page 1 and says she left in October, then the personal exemptions, deductions, RRSP contribution limit etc. have to be pro-rated to that period. If she had Canadian-source income subject to reporting after she left, she needs to file a non-resident T1 for that portion of the year, similarly pro-rated.
Or alternatively she can file a resident T1 for the whole year, and claim a foreign tax credit as explained in the general guide for the T1 for the
UK tax paid during that period. Working out the UK tax for that period can be quite awkward. If she does it that way, she puts down that she left on 31st December then that's it. She must sever residential ties to Canada and notify the bank that she's non-resident. If she's got an RRSP or TFSA that is not recognized as a tax shelter in the UK so any interest or distributions will be subject to UK income tax.
Steve.