Employee contributed the maximum $10,500 to a 403(B). He terminates, and is hired by a company that has a 401(k) plan in the same calendar year. Do the 402(g) limits apply to both plans which would prevent him from contributing to the 401(k) plan?

Deferrals under the 403(B) plan don't prevent deferrals under the 401(k) plan, but deferral of more than $10,500 for the year, taking into account all elective deferrals under all 401(k), 403(B), SIMPLE and SEP arrangements, has some adverse consequences. Depending on the terms of the plan, the person may be able to elect a refund of the excess aggregate amount, or the person could suffer the tax consequences of the excess deferral.

I would be careful about preventing an employee from deferring unless the plan document blocked the deferrals and the administrator was sure of the facts. Nothing wrong with educating the employee about the rule to try to influence the election.