It's not up to 123.com if any previous email address will continue to work.
If the friend has an existing free email account such as Gmail / Hotmail / Yahoo their account will continue to work.
If the friend has an account from an ISP such as Verizon / Comcast / Frontier / Time Warner etc then the account may or may not work after they terminate their service.
The friend needs to check with the existing email provider to find out if that provider will let them keep the email account after their service is terminated.
This is one of the reasons general free accounts such as Gmail are better to use than ISP accounts you could lose if you change ISP's such as Time Warner. If you friend got their own domain name and didn't use ISP accounts at all that would be even better!
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