Seems like tortoises prefer raw over cooked foods. I used to give my redfoot many different types of fruits, but I never offered her boiled sweet potatoes. We'll see what others say, but if it were my redfoot, I would only stick with what she could eat raw. Given that tubers (potatoes, sweet potatoes, yams, etc.) are so firm, I would not offer them to my redfoot tortoise, either raw or cooked.
Sweet potatoes are almost a super-dose of Vitamin A. They also have many, many other minerals and vitamins to offer, and most of them in abundance. I commonly feed sweet potatoes to my red foots, and they seem to like them. To prepare, I grate it and mix it in with their greens. I've heard some do boil it til its soft, so you have just a big mushy potato they can easily bite, but I grate most things so my tortoises can't be picky.
In short, its perfectly fine to add and can be safely added to the 5-10% vegetable part of any red foots diet.
I have fed my torts potato before but they didn't care for it much. My torts do liked steamed veggies. I fed them steamed cauliflower and they loved it. I think things like cauliflower or broccoli should be steamed because they are just so crunchy for a tort.