Great info in Katana's post. I have a small TAG who is almost 5, she is very aggressive toward other parrots. I call her my stalker. She is the boss although my CAG outweighs her by 200 grams. Introducing the two parrots to each other in your home while your friend is there is a great idea. It will let you know if they will tolerate each other's company. Talking about the fact that the zon is visiting and then the zon leaving with his/her owner will help your grey understand what visiting is. A couple or three visits and then the zon leaving let's your grey understand what visiting means.
Zons are great parrots, you may find you can't live without one so beware, they are addictive! LOL!