Short answer: The employer plan pays first for the first 30 months of Medicare eligibility based on ESRD, regardless of employer size or whether the person is still working; Medicare pays first after that.
During the first three months of dialysis, before Medicare starts, the employer plan is the only payer. Then the 30-month coordination period begins, with Medicare paying second. Each new period of ESRD-based Medicare entitlement starts a new 30-month period.
Because the plan pays first, a person whose plan has little cost sharing may reasonably delay Part B until the 30 months end, and Medicare says the Part B premium will not be higher for that delay. Someone with a large deductible or coinsurance may want Medicare to pick up those costs. If the plan’s benefits are reduced or end during the period, enroll right away.
For employers, this is a Medicare Secondary Payer rule, so the plan cannot push an ESRD member onto Medicare as primary early. Medicare.gov also notes that COBRA, which is normally secondary to Medicare, is the exception for someone with ESRD.