Any metal in excess is dangerous when ingested. but keep this in mind:
The reason they say aluminum dangerous is because it's been linked to Alzheimer's. While the people studied with Alzheimer's have a much higher content of aluminum in their bloodstream this doesn't mean that the aluminum was the cause, only that they retain it. Classic case of correlation doesn't necessitate causation. (One thing that the news is always guilty of putting out there. Their logic seems to be that study shows correlation between A and B, therefore A must cause B.)
Looking at aluminum oxide, it basically instantly covers the surface of aluminum when exposed to oxygen, so even when you have foil out of the box, it already has a layer of aluminum oxide. Another property of aluminum oxide is that it is very hygroscopic. Basically meaning that if it does come off the surface of the foil (which I find fairly unlikely, most of the foil is perpendicular to the flow of air) it will most likely absorb the water and become part of the water. I don't think the temperatures we are dealing with here have much to do with the production of aluminum oxide anyways, so overall I think it's not a huge issue.
After a full session with just straight aluminum foil, the foil generally seems unchanged for me (no obvious oxidation, which would be seen as a dull patch). If the foil does in fact oxidize it would probably cling to the shisha itself and the water. The amount that you will receive (if at all) is most likely marginal. I wouldn't worry about it - save yourself some money and just use the aluminum foil.
On another note if someone has an infrared thermometer and wants to measure the temperature on different types of coal when they are the most hot, I'd be interested to find out what kind of temperature we are looking at. the temperature to burn charcoal is much greater than the temperature to melt aluminum and obviously that isn't occurring so it must be much less than that. Aluminum doesn't get cherry red like steel so it's obviously hard to observe that. But then again, it shouldn't affect the oxidation of the aluminum.
If I am wrong tell me, but I'm pretty sure I read correctly.