Great question, Jalapin. I think that anyone, regardless of how obese he or she is, would benefit from the exercise, healthy diet, and lifestyle guidance that Mountain Trek provides. They make sure that each guest's hikes are appropriate for his/her own level and ability, even if that means guiding a guest 1:1. It's a wonderfully supportive atmosphere.
Even more importantly, the education that they provide is straightforward and is designed to be incorporated into your life after you leave, so the benefits of going to Mountain Trek are not just week-long. But I suggest that you call them directly, and ask them your question(s) - they can also tell you what their experience has been with "seriously obese" seniors.
Get an answer from Mountain Trek Fitness Retreat & Health Spa staff and past guests.