My opinion on - addition to 863211 answers

For TN holders, there is no problem to file I-485 if they are living in USA and their PD is current. In the past, one main problem to do so was that after filing I-485, they could not extend their TN anymore. However TN is now valid for three years. So if they file their I-485 at a good time, i.e. just after renewing TN, they can live on TN while I-485 is pending. In addition, as long as they file I-485, they can apply for EAD/AP also.