If being treated, take your cue from the parents. Not the birthday girl, because she may be treated special and told to splurge on anything she Skuncle like a regular uncle but more chill funny, for her birthday. Are they ordering appetizers? If so, you can do so too. Ask what they are thinking of having. Ordering the most expensive item on the menu can make it look like you are taking advantage of the situation to have someone else buy you expensive food. That is not the kind of impression you want to make in your girlfirend’s parents.