You could give her as a gift but not birthday gift. I mean, on her birthday it has to be special but if you give her that on her birthday she might not take it well. Try to talk to her and say that she could be so much more attractive if she had white teeth. Don’t be so negative about it, she might take it well but having this “surprise” as a gift would not be very pleasant.

Send her the gift as an anonymous person. You need not make your friend know that it was you who send this gift. By receiving this gift she’ll understand that there is some problem with her teeth.

If I were your friend then I would have been surely offended. This will be like a shock to her. Give her some other gift but not this Go smile stuff. You’ll ruin her party. Your friend must be aware of the fact that she has yellow teeth, so please don’t tell her.