It is actually possible to have a respondent start offline and resume a survey online. So one option would be to set up the survey with a unique password. Have the respondent begin the survey on the offline device with that password. Get to whatever the cutoff point is and have them stop on the offline device. Then, sync the offline device.
As long as the data gets synced, the respondent could then access the online link for the survey, type in their password, and they will resume where they left off. You could also email the link to the respondent and embed the password so they did not have to type it in and risk making a mistake.