If your friend had not yet decided whether to go, you could have said "If you go ... you will eat ..."
If you saw him the day after the lunch you would say "If you had gone ... you would have eaten ..."
I could not understand what you meant by "right by the way I met him now". Did you mean that you had just met him in the street and asked him about the lunch?
I'm also not sure what information you were trying to impart. Does your friend eat more chicken than anyone else when he goes out to lunch? Had he already had lots of chicken in the morning and you were suggesting that he would be able to eat even more chicken if he attended the lunch?
Student or Learner