To answer your quesiotn: No, dual booting 2 (or more) OSs is not a violation of warranty.
As others have stated though that you should not be dual booting the pirated OS, you should be running the proper OS.
Either take the laptop back to the tech and say that you have a proper license of Win 10 and that he needs to put the proper OS on your laptop (ya know, since what he did is illegal).
Or you will have to get recovery media from the OEM, now this may already be on your computer in a hidden partition (assuming the idiot "tech" did not delete it).
What is the make and model of your laptop?
Did the laptop come with windows 10 pre-installedo on it?
If the laptop is under warranty why did you take it to tech, was it just a...