However, it could also suggest a strong rental demand if the units are consistently occupied.

I have had some clients flip from out of state, it worked as I gave them my contacts but its risky and not something I would suggest.

I will take this and do my part and look into these suggestions.

Here are a few initial questions that I have: - I'm wondering if there's a recommended corporate structure for these type of business activities, focused primarily on both short-term and long-term rentals.

I need suggestions on crafting a sub 2 deal.  

Suggesting that owning property in Puerto Aventuras is better than buying in Playa and Tulum because some areas in the latter cities is akin to saying it's better to own property in a little town in Upstate New York than in New York City because there are crime problems in some areas of New York City.

I would suggest you spend a little more time than just a weekend.  

Suggest you ask a LOT of questions and take good notes, so you can copy their systems.BTW - many PMCs won't help you because they don't do rent by room.

I have listed the upper unit for rent 2 months ago and the lower unit 1.5 months ago, I am wondering if anyone has advice on what I should do differently?

It's possible the deceased father agreed to let her manage the property and keep the rent net for her troubles, but the title/ownership remains in him so his heirs would inherit it as a legacy.I suggest your friend familiarize himself with the statute of frauds which I believe requires agreements regarding real estate be in writing to be enforceable.