If you're looking for just one person, then there are a lot of skills this person should have!
- Good illustration skills (eg. for character-design)
- Good knowledge of computer based creative-tools, preferably knowing technical requirements for games (you don't want to scan and crop some drawings, do you?)
- Person should have a feeling for good UI-Design
- Good knowledge of typography and readability
- Must also be able to create animations and effects
- Able to create icons (logo) for a game (note, that this is an entirely different skill-set sometimes)
- Should be able to produce a consistent look and feel within given requirements
I think these are some common requirements. Each one of these requirements could be filled by a single person, so finding one that excels at all of them will be near impossible. You should therefore set some priorities what's most important to you.
In addition to the above, there's also the usual skills that should be there, like good communication and teamwork skills etc.
Update: Since you extended your question, here's an updated answer. So you have a skilled friend and you would like to give him advice on what to learn, so that he can participate in your game-project.
I'd say it highly depends on the art-style you're going for. If he's a painter and you're creating a game with painted assets, even better. Then the main thing he has to learn is to scan the paintings and mask them (eg. apply define the transparent and opaque areas of the sprite). Also he should pay attention to scale. It doesn't make sense to paint lots of details when the assets are going to be shown with only a few pixels in size.
Knowing tools like Photoshop/GIMP and Illustrator/Inkscape is a must and something he/she should learn first.