He wants the sprites to be High Quality so think in the terms of fighting games. It's going to be a sidescroller.
Either he's not very experimented as a game creator, or he's not really a friend

for asking you what will amount for several months of work. Lower resolution / level of detail is much more realistic project. (Note: everything should use same resolution and be pixel-aligned, otherwise it will look jarring)
A typical game with 4-6 frames for each movement (which is readable, but not very smooth) quickly ends up in hundreds of drawings because of the number of characters and their movements.
The character size relative to the scenery, character speed, and jump height are extremely important gameplay elements. They should be determined very early, because if you change your mind later, a lot of graphics can go to the scrap. Screenshots of existing games (the 2D Castlevania, etc.) can be helpful to get an idea of how much space these games leave around the player character. Whether a character can crouch to avoid shots is a critical piece of information.
A well-rounded artistic background will be extremely helpful, but if you don't have experience yet with animation, don't underestimate the trial and error that you'll go through. Still images can look great while they animate poorly, and this cannot be fixed up at the end: Animation and timing must be double-checked before you finish and "dress up" your character, otherwise you'll waste a lot of time redoing the same sprites.
The "bread and butter" of a sidescroller main character is his run animation : This loop of 6, 8 or more images is the one that the player will see most often. You can already have a test run (hah!) of this one. Even with just sketch lines, it's what the game creator can use to check that the character moves fast enough, etc.
In most cases, you will have complete freedom of timing ("this image should be there for 100 milliseconds, this one for 333 milliseconds etc"). This means you can always insert one frame between two if a movement is not smooth enough.