In a memo dated, January 29, 2001, Michael A. Pearson, Associate Commissioner, Office of Field Operations, outlined the requirements for admission of an individual at a Port of Entry in the circumstances described in your question. The memo states that an H-1B applicant for admission, who is no longer working for the original petitioner is admissible pursuant to portability provisions of AC21, as long as he meets certain requirements:
  1. Is otherwise admissible
  2. Is in possession of a valid visa and un expired passport
  3. Satisfies the POE officer that he was originally admitted as an H-1B nonimmigrant
  4. Presents evidence in the form of a receipt that a new H-1B petition has been filed and is pending with the Service center.
I recommend that the employee carry the original receipt from the USCIS when he visits abroad. A recently dated job letter will also help.