You'll probably have pho since it's like the classic dish, which I think is a bit overrated. But Vietnamese food, as a whole, is probably my favorite foreign cuisine. Spices and herbs and key; even if you order something that sounds simple, the way it all works together with the diverse, tangy flavors makes for a wonderful taste. That is, as long as you're going to a pretty legit Vietnamese place, which since your friend is Vietnamese I'm assuming will be the case.

I was lucky enough to have it in Vietnam, but even being there for two weeks wasn't enough to try everything I wanted to. I'm sure I'll be going back again sometime to rampage through their cuisine like Psychotic in a dong factory. Enjoy!