Your friend must apply for a visitor visa. You do not need to send him an invitation letter but you can do so if you wish. A visitor visa can work if your friend can demonstrate he will depart the United States after a brief stay. This requires showing ties to his country.
A J-1 visa is a completely different matter. He would need to be sponsored by an employer for a J-1 program. There is not enough information to determine if this is appropriate for him.
You read more about nonimmigrant visas at http://myattorneyusa.com/us-nonimmigrant-visas.
Answered on Mar 07th, 2016 at 5:08 AM