Thank you for your question and patience, I’m Tom and I’ll try to help you.
Your partner will have to apply for her new passport before she is able to naturalise.
This is because a full valid passport is required to be submitted with the AP1 application form to apply for naturalisation. If the passport has expired then it is not valid and the application would be rejected and the application fee retained.
Your partner should apply for a renewed passport. However, she will likely still require the old passport as evidence of her time spent in the UK through the period of time that she will be relying on to show her eligibility. Therefore, she would request from her government’s department renewing her passport that she is permitted to retain the old passport following renewal.If she is required to first send the old passport off during the reneweal period, then I would suggest that she attends a local solicitor to ask for a certified copy of all pages of her passport, the solicitor will certify the copies they provide her as true copies of the originals. These may come in handy if the old passport is lost in transit.
My goal is to provide you with a good service. If you feel you have received anything less, please reply back as I am happy to address follow-up issues specifically relating to your question.