Advice on dealing with selling a timeshare in Mexico (Vidanta)

I am seeking advice on dealing with selling or canceling/non-renewal of a timeshare in Mexico. My father and his wife are in their early 80s and have had a timeshare through Vidanta for about 15 years. Vidanta seems to have very aggressive upselling tactics whenever timeshare 'owners' use their weeks, and my dad had bought into higher level of their services compared to his original contract. As such I suspect the original investment, which might be the only 'sellable' option, is a fraction of what he has paid to use the timeshare. 

Now that he is no longer able to travel easily, they would ideally like to sell their shares. As an aside, unfortunately, I believe they were recently scammed out of several thousand dollars by a company pretending to help them sell the timeshare. I hope they have learned their lesson not to give any money upfront to sell something, and please keep an eye out for these scams with your own families!

My questions are these: 1) Does anyone here have experience dealing with selling/releasing shares of Vidanta timeshares back to the company? 2) Has anyone on here heard of someone successfully selling a Vidanta timeshare to a buyer, and have advice about how/where to do that? 3) Should I recommend that they get an American lawyer involved, at least to review their contract or even negotiate for them with Vidanta, and if so do you have a recommendation for a lawyer with timeshare/Mexico/Vidanta experience? 4) My dad is under the impression the only way he can sell the timeshare is to go to Mexico and talk to Vidanta on site, which just seem like another way that Vidanta will try to confuse an elderly person, upsell them, or give them no money for their timeshare? 5) Does anyone know the process of 'walking away' from a Vidanta timeshare or other Mexican timeshare, if that's what they need to do?

Unfortunately they could really use some money from selling this timeshare to help pay for retirement. My understanding is that most people only can sell any timeshare for a fraction of what they paid ... but I fear that Vidanta makes the fees for transfer very high, such that they may just need to walk away.