They were correct in that you need a loop vent. They should also have told you that a 2″ pipe is the very minimum that will work for a loop vent. A loop vent is an inverted “U” that rises as high as possible in the sink cabinet, but definitely higher than the bottom of the sink. Both legs of the “U” tie into the horizontal drain line, but the one designated as the vent, also has a tee connection that goes to a normal verical vent line. Some jurisdictions will allow the line off the vent to be eliminated if the horizontal pipe is 3″ or larger.