hi there.
First off, yes, you can leave the UTH on at all times, day and night. I would recommend getting a thermometer with a probe so you can measure the temperature of the ground over the UTH, and it should range between 88-92 degrees. The other side of the tank should be a bit cooler, about 75-80 degrees.
I think you may want to reconsider using sand as a substrate. It's not the safest choice as it could cause impaction in your friend. The slate tile is great; other good choices are repti-carpet and papertowel, even though they aren't the most attractive.
Hope this gives you some help... 