Medicare Payments for Non-US Citizen living Overseas?

I'm a US citizen. My wife is not. Years ago she had a US green card, but she returned it after we moved overseas years ago. We now live year round in Asia.

Both my wife and I have paid into the US Social Security system long enough to qualify for Social Security in the future. My wife could receive full Social Security starting at age 66. I could at age 66 and a half. And we’ve also both qualified for Medicare someday. My questions concern Medicare.

My wife is rapidly approaching retirement age, and her most recent statement from the Social Security Administration had a reminder to "be sure to sign up for Medicare three months before turning age 65... (or) you may have to pay a late enrollment penalty."

As I mentioned, she’s not a US citizen and she’s also no longer a green card holder. And it’s likely we may never return to the US to reside. So MUST she apply for Medicare and start paying for Medicare B at age 65? (Or, as a non-US-citizen and a non-resident living outside the US, could she even apply for Medicare at age 65 if she wanted to?)

And what if later on (perhaps at age 70) she and I were to decide to move back to the US, and she got another green card. Could she then apply for Medicare? Would she be hit with the late penalty?