The employee cannot be denied coverage. When it comes to the spouse of the employee, there are two things that can happen.
For one, the company can keep her on for the full 18 months, which at that time, she can contact the state Department of Insurance, and they will put her on a HIPAA Plan (just as expensive as COBRA).
Secondly, if they tell her that when he goes, she goes with him, she will need to contact the state Department of Insurance, and within 63 days, obtain HIPAA plan coverage.
My suggestion for him is to get a Medicare Supplement or Medicare Advantage Plan, alongside Medicare Parts A and B. I would also suggest he get Part D coverage, standalone, within the 7 month period of his birthday month (3 months prior, the month of, 3 months after). If he waits beyond that period to pick up Part D, he will be penalized 1% per month for each month he does not have it.
Your husband should talk to his employer. To my knowledge, you cannot be denied from COBRA for a pre-existing condition. COBRA only lasts for 18 months, if I remember right.