The employee will become a resident of US if he is physically present in the U.S. during a period that is at the least:
1. 31 days during the current year, and
2. 183 days during the 3-year period that includes the current year and the previous two years, counting:
all of the days you were present in the current year, and
1/3 of the days you were present in the first preceding year, and
1/6 of the days you were present in the second preceding year.
Once the employee is resident his worldwide income is taxable in US. So the salary from Italy may be taxable. However, he will get a credit for tax paid to the foreign country and hence double tax would be avoided.
Even considering the Double tax treaty (link attached) the above will apply.
Let me know if you have any question.
Please note: This advice is provided with the understanding that all the relevant facts have been provided by you. Any change in facts might affect the advice given and hence may not be relied on in such cases. Nothing contained in this reply was intended or written to be used, can be used by any taxpayer, or may be relied upon or used by any taxpayer for the purposes of avoiding penalties that may be imposed on the taxpayer under the Internal Revenue Code of 1986, as amended.