The short answer is no, you cannot use Medicare
The short answer is no, you cannot use Medicare here in Colombia. The long answer is beyond the scope of a response. International health care policies are a good option, but, as always, the fine print is your friend. For instance, many international policies will not insure you within a certain number of miles of your 'home address,' so don't let that codicil bite you. In our case, with a home address (for now) in Ohio, we can't use our policy within 100 miles of Columbus, so we must acquire an additional policy within that range (I know, it's nuts) or risk going uninsured.
The really good news about all this is that many health care systems abroad are much, much cheaper than that found in the U.S., for example, while retaining the quality found there--or exceeding it. My wife needed a full panel dental x-ray when we arrived in Medellin, and she received it in one hour, with no appointment, for about $6.00 US. (You read that right, no need to wipe your glasses.) No co-pay. No insurance interaction.
So one answer to the question is to acquire emergency or major medical coverage only, and pay out of pocket for the rest. Not exactly an answer to the original question about Medicare use, but an elaboration on options. Another item to consider is Tricare, if the retiree is eligible for military retirement bennies. Tricare can be used (I'm told) by paying for the expenses, then submitting paperwork to Tricare for reimbursement. Disclosure: I've not gone this route. I've only been told it's possible, so good luck.
(Photo: Medellin River, Christmas 2004. Credit: By Aliman5040 - taken by Alejandra Zapata (my cousin), CC BY-SA 3.0, Wikipedia.)