Fine for operating not single-pilot certified jet with one pilot
According to FAA Order 2150-3B in Appendix B (penalty guidelines) the recommended penalty for "Failure to comply with operating limitation" is a 30 to 90 day license suspension. That is what I would expect would happen in real life if the FAA investigator thought that the pilot had no good reason for operating without a co-pilot. If there was some good reason and the pilot's record was clean otherwise, they might change it to a warning.

I disagree with the other answers that claim it would be quickly reported. Unless an informed passenger noticed it and reported it, I doubt it would get reported by anyone else. The fuel guys have no idea what operating limitations are or who is operating the plane. In many cases, crew and passengers are long gone by the time the fuel truck pulls up. For the fuel truck to deal with just one guy or to fuel an unoccupied aircraft is commonplace. For all the fuel truck guy knows, there could be crew inside the aircraft doing stuff. He has absolutely no idea what the operating situation of the aircraft is.