I'm not familiar with that model. I'm guessing that it is a two-piece? (tank comes separate from the bottom). If so, then there are two (usually) bolts that hold the tank to the bottom. They have a rubber washer/sleeve as part of it that keeps the thing from leaking once the toilet is assembled. If he tightened one side up as tight as it should be, then tightened the other side up, rather than following the instructions that say to tighten them up evenly, it would tilt to the side that he tightened up first. It takes maybe a minute or two longer to do it the right way.
When you say tilted, is the tank tilting to one side, or is the whole thing tilted? That makes a difference. If it is the whole thing, does it rock back and forth? Did he use any shims to make it sit without rocking? Is your floor level?
My guess is he was sloppy putting the tank on, but it could be other things as well. If that is the case, (he should fix it) you can fix it yourself. Course, if you break it, you have no recourse. See if there is a bigger gap on one side of the tank vs the other.