What a beautiful Aldabra -- and it was so good of you to rescue him!
I've had a look re the Rambutan, and I'm finding it a bit difficult to decide how good or bad it would be for him to have the leaves and fruit of the Rambutan (Nephelium lappaceum) tree so readily available.
The diet of Aldabra tortoises is closer to that of the Sulcata and Leopard tortoises (large amounts of grass) than it is to Mediterranean tortoises or Redfoots, but they do eat other leaves and occasionally fruit -- but not often. Your Redfoot would probably be very happy to eat the fruit that has fallen from the tree, and the Aldabra could also probably occasionally have a fruit, but his digestive system is less suited to it than a Redfoot is and too much would upset his digestive system.
Regarding toxicity, it's really difficult, because most of the studies are done regarding the fruit in relation to mammals. It's known that the seeds of Rambutan fruit are toxic when raw, as they contain saponins, that can affect motor skills and red blood cells. Here is what we say about Saponins on our website:
"Saponins are a group of steroid glycosides which are capable of causing anti-nutritional effects. They are found in many plants including Alfalfa, Vetch, Spinach and trees such as Maples and Horse Chestnuts, as well as most legumes and soy products. Cold-blooded animals such as fish, insects and reptiles are particularly sensitive to saponin glycosides, and high levels can speed up haemoglobin degeneration, irritate mucous membranes and act as an appetite suppressant. Saponins often have a bitter taste. Although all saponins are not bad it would be best to feed with caution as only a small part of a tortoise's diet."
So most studies on the effects of Saponins concern mammals, but we know that reptiles are more sensitive to saponins than mammals. If a tortoise ate the seeds of the fruit, they would most probably just pass through their system and out the other end and do no harm, but I couldn't guarantee that. The fruit itself, probably contains too much sugar for the Aldabra's digestive system to cope with, but the odd bit won't hurt him.
Regarding leaves, there was very little that I could find, although I did find this article:
http://devel.uad.ac.id/farmasi/wp-conte ... ice-...pdf
As you can see, the mice fed the extract of Rambutan leaves did show toxic effects, but they didn't die. Also, they were being fed extracts that were probably much higher in concentration than anything a tortoise would eat (although I couldn't guarantee that). And of course they were mice, and not tortoises.
So I can't reach a definitive conclusion on this. Is there any way that your Grandmother could fence off an area that is not so close to the tree, and regularly clear fallen leaves and fruit from that area? The odd nibble probably wouldn't harm the Aldabra, but if the ground is covered in leaves and fruit and he eats lots, then I couldn't guarantee that he would be OK.
Nina