Short answer: If they return to Original Medicare, federal law guarantees Medigap Plan A, B, D or G (C or F for people eligible for Medicare before 2020); they can apply as early as 60 days before the Advantage coverage ends and no later than 63 days after it ends.
The right applies when the Advantage plan is leaving Medicare, stops serving the area, has a significant network change, or the member moves out of its service area. It exists only if the person switches to Original Medicare rather than joining another Advantage plan. The Medigap policy cannot start until the Advantage coverage ends.
The menu is limited: not every plan letter is guaranteed. Plans C and F are available only to people who were eligible for Medicare before January 1, 2020; people new to Medicare on or after that date get Plan D or G instead.
Keep the termination notice and the postmarked envelope: insurers may ask for proof of the guaranteed issue right. Outside these windows, a Medigap insurer can ask health questions and turn the person down.