Clubman is correct - you have no statutory entitlement to job-share so the change in your partners situation may have an impact on you. For example, your employer might choose to recruit another job-sharer to fill this vacancy, or they may choose to allow you continue job-sharing without a partner.
I spent 4 years some time back job-sharing on my own, before returning to full-time employment.
At the end of the day, it will depend on your employer and the terms and conditions of your employment. At this stage, your partner is just 'talking' about returning to full-time work, this doesn't indicate a commitment one way or another so I wouldn't be concerned yet.
It might be wise however to enquire from your employer as to your position if this were to happen.
CMCR.