"Unsafe burning of buildings: Sometimes people decide to get rid of old sheds or dangerously dilapidated buildings by simply burning them down. This is legal, provided you do it with the knowledge and cooperation of the fire department. If you burn an old building in a reckless or dangerous manner, you are guilty of a Class D felony. This can be punished by up to 25 years in prison."
I doubt that he will get the full 25 years, but my guess would be anywhere from a week or so to a few months.
If the court system is anything like it is here in Texas (in some parts), don't be surprised if they will try to tackle on other charges to extend his sentence.
Such as invasion of property, breaking and entering, those types of things.
Good luck to him.