Depends on the circumstances, who you are with, who she is with, the venue etc. The general rule is to uphold the mutual agreement for discretion and say and do nothing. However depending on your relationship with the girl eg. is she a regular, do you communicate out with bookings? In this case a discrete nod or smile may be acceptable. It would be hard to imagine an occasion when a full blown "hi how are you?" would be appropriate as even if you thought nobody was around, you can't be too careful, unless you have both agreed a cover story should the situation ever arise. But I doubt many WGs would do this.