If they pay on the day it expires it should be ok. Is it an unlimited plan or a regular/smartphone plan that has service days/talk/text/data in separate "buckets"?

Tracfone Terms and Conditions:

REFILLING YOUR ACCOUNT BEFORE YOUR SERVICE END DATE. If you do not purchase and add airtime prior to the Service End Date, your Tracfone Service will be deactivated on the Service End Date and you will lose your phone number even if you have minutes remaining. To prevent this from occurring, please keep your Tracfone Service active by purchasing and adding an Airtime Cards on or before your Service End Date. Tracfone reserves the right to deactivate any phone for which there is no voice, text or data usage for a period of six (6) consecutive months for smartphones and twelve (12) consecutive months for other phones. If your Service is deactivated, your Service can be reactivated by purchasing and adding an Airtime Card. Once reactivated, your phone may be assigned a new phone number. Airtime which remained unused at the time of deactivation will still be available if Service is reactivated within sixty (60) days from the deactivation date, otherwise any unused airtime will be lost.

Ive had my service days expire by a few days several times and still maintained my balance and phone number when I added service. I would try not to do it very long but theres some grace period.

Mine went out of service and was out for only a week because I had to wait to get paid, and by the end of the week, they had reassigned my phone number.

Now a Mexican man has my old phone number and has managed to hack multiple social media accounts of mine by having my old number.

I screwed up one time and forgot when service ended. Fortunately I had friends send emails saying my cell service wasn't going through. Once I realized what happened I was able to purchase a plan and rescue everything the next day, even though it was 3x what I normally pay.

My phone number and all the minutes/texts/data was still intact.