Short answer: Yes, if they have Original Medicare and the employer, retiree, COBRA or union plan that pays second to Medicare is ending: they can buy Plan A, B, D or G (or C or F if eligible for Medicare before 2020) within 63 days after the latest of the coverage end date, the date on the termination notice, or the date on a claim denial that first showed the coverage ended.
Someone on COBRA can buy the Medigap policy right away or wait until COBRA ends. Keep letters, notices, claim denials and the postmarked envelopes; insurers may require proof of the right with the application.
This window is 63 days, not the six-month Medigap open enrollment period. Someone who retired after first taking Part B while still working has usually already used up the six-month window, which starts the first month of Part B at 65 or older, so this guaranteed issue right is often their only route to a Medigap policy without health questions.
Other federal rights: the Medigap insurer goes bankrupt or coverage ends through no fault of the person, or the person leaves an Advantage plan or Medigap policy because the company broke the rules or misled them; each also allows Plan A, B, D or G within 63 days.