Yeah ICHRA requires a (mostly interesting/fun) shift in many ways thinking. One is that the ICHRA itself is the group health plan. More importantly for these purposes, it's a non-excepted group health plan subject to the HIPAA portability rules, including special enrollment rights. The underlying individual policy is not an ERISA plan and not connected to the employer in any way.
With that in mind--when an employee loses eligibility for the ICHRA (from termination of employment in this case), it's a HIPAA special enrollment event for the spouse in the same manner as if the employee had lost a traditional employer-sponsored group major medical plan. That event gives the spouse the right to enroll the employee and spouse in the spouse's plan. The underlying policy is irrelevant here.
More details: https://www.newfront.com/blog/hipaa-special-enrollment-events-2
Even with traditional employer-sponsored group major medical plans we have an issue of how to substantiate the loss of coverage since the end of HIPAA certificates of creditable coverage back in 2014. The most useful is usually a letter from the employer showing the period of coverage. In this case it would be the period of ICHRA coverage.
More details: https://www.newfront.com/blog/documenting-prior-coverage-after-hipaa-certs-2
Here's a slide summary:
2024 Newfront HIPAA Training for Employers Guide