What should be done if an employee needs to leave Thailand without canceling their Non-B extension?

You don't cancel the Non-B extension for her, SHE goes to the immigration office with termination paperwork before she leaves and cancels it 😕 If she can't do that she can just leave.

𝐖𝐡𝐚𝐭 𝐚𝐫𝐞 𝐭𝐡𝐞 𝐢𝐦𝐩𝐥𝐢𝐜𝐚𝐭𝐢𝐨𝐧𝐬 𝐢𝐟 𝐬𝐡𝐞 𝐥𝐞𝐚𝐯𝐞𝐬

None, she can stamp out on the current extension

BUT

when/if she comes back she needs to tell them NOT to stamp her back in on the extension w/re-entry so she can go cancel it after she gets back at the immigration office.

Tell her to let passport control know she wants to enter visa exempt

She will get fined for not having cancelled it when she goes to cancel it once she's back though.