Open Enrollment Overlooked Dependent

Hi. An employee just notified us that he forgot to include a dependent child on his dental coverage when electing the plan during open enrollment in November 2019 (the child was selected to be covered under the medical and vision plans). We are wondering if there would be any administrative type reason to allow the child to be covered, retroactive to 1/1/2020 or if doing so would violate Section 125/ERISA rules? There has been no qualified life event or status change to permit a mid-year enrollment. This is purely an issue where the employee just realized their mistake 7 months into the plan year.

Thanks!