Of course, this would be considered safe, the underlying question is, why? Boiling potatoes with or without skin, whole or cut up will boil and cook well. Wrapped in foil and boiling would be uncommon. The physics of this method would suggest that it would take longer to cook for two reasons: It takes more time for the water heat to reach and ‘cook’ the inside of a whole potato, and foil would add yet another barrier and in fact, would likely cook uneven. The best way to boil potatoes is to cut into medium-size, roughly equal parts first. If you wish to serve inside the foil after preparation for aesthetic sake, this is certainly an option.