Will seats from a 2000 silverado 1500 fit in a 2011 Gmc sierra 1500?

They might. I'm not sure if they changed the floor pan design between the GMT800 and GMT900.

Even if they did, the 2011 probably has passenger presence and maybe even seatbelt switches which would have different connectors for airbag and other telltale defeating purposes. so he should be prepared to deal with those before trying to fit them. They may also not match color wise depending on interior colors between the two.

Good point!