Whats the best digital art form for video game programming?

There is no such thing as C+. You are probably talking about C++. The second + sign is not a typo. Also, as I recall, the iphone doesn't take C++, rather you use a different language called Objective-C which is superficially similar to C++. For doing game artwork, you should be familiar with basically everything relevant to it: Bitmap editing, 2D vector artwork, 3D artwork, and how to combine these to get good results. You don't need to be intimately familiar with any specific piece of software, so long as you have a deep enough understanding of the general principles that apply across multiple platforms. And of course, be sure to practice your artistic skills, experimenting with different styles, tones and montages. In particular, practice with things like human proportions, architectural artwork and landscapes, since these come up often in video game art.

The thing about using Photoshop is that although there are specific techniques used for hiding repetitive textures and the like, traditional art skills WILL NOT fail you. The better you are at drawing, painting, the easier it will to transfer those tools. Your friend will also have to tell you how he wants those images created, since they'll be specific to the game engine he writes/works with.