I think its a waste to order and pay for food you're not going to eat. I can't even believe I just read that.

do they have their menu online? look it up, pick something you can have, and get it. If not, see if they have a healthy choice section (most do) and use it. drop in the day before and have a look at the menu, then look up the apporximate caloric values online. or eat half a meal before you and have a side order at the restaruant. or have a cheat night and let yourself have something you normally wouldn't (maybe ask your friend to split a dessert). there are more options to you than wasting food.
fighting the temptation to overeat is all part of this process. gotta face it sometime.