I don't think beating around the bush, or fobbing it off onto the waiter is the way here. I think you have two options: either you keep quiet and both you and your BF order off the menu, or you lay it out bluntly on the way to the restaurant:
'Friend, BF and I are going to order the banquet tonight and we have to insist that you not eat the banquet food if you order off menu. It makes me really uncomfortable when you eat the banquet without paying for it.'
If she insists that she only takes a little, or that it's not a big deal, rinse and repeat: 'Nevertheless, it makes me very uncomfortable when you do that, so please either order the banquet or keep to your own plate.'
And if she agrees and then takes some anyway, 'Friend, I thought we discussed it earlier. If you want the banquet then we will order it for you.' and then you can call the waiter over and have him add it to the bill.
I also think you need to grab whatever your favourites are and take your share before she does, just in case.