Is per diem taxable to a contractor once they have been at the same job location for more than one year?

If so, can they have a break of service for let's say 2 weeks then return to that same location and not be taxed on per diem?

Yes, it is taxable.

The two-week break is irrelevant. The break could be for three MONTHS and it would STILL be irrelevant, because the IRS would see it for what is was, an attempt to evade the taxation of the per diem.