My client asks me about using SQL 2019 evaluation in his live environment until it expired
No that is not allowed. I would quote from Microsoft User License Terms
EVALUATION USE RIGHTS. If you acquired an evaluation version of the software, then the EVALUATION
USE RIGHTS described in this section apply to your use of the software:· You may use the software only to test, demonstrate, and internally evaluate it.
· You may not use the software in a live operating environment unless Microsoft permits you to do so under another agreement.
· TIME-SENSITIVE LICENSING. The evaluation license you have for the software will expire after 180 days. Unless the software is validly licensed, you have no right to use the software after the time permitted for evaluation.
Similar information is present in This Blog