We did this excursion in March of this year, and it was already pretty hot and humid! However, I found the tour to be nicely paced and there were several places where you could sit. If the AC on the bus is working, I don't think your mom will have much trouble with it. The walking path was MOSTLY even. If it's a concern, a cane might be good, might be overkill.
After the ruins, the stops are short with not much walking. Chances to shop and buy drinks. Not too strenuous at all. I suggest sitting on the left side of the bus if you like ocean views during part of the drive.