It sounds like your partner is just getting out of school and doesn't have much experience yet. Typically, artists in that position can expect to be making around $15-$20/hour doing freelance jobs (as its super hard to get a job in art straight out of school, or at all). I'd suggest that (or possibly lower) as a payment base for your situation, especially since they are also receiving a stake in the project's profits.
Personally though, my partner does all of the graphics and we share a 50/50 split (I handle programming, music, and marketing, he handles graphics, writing, doing the bug testing, and advertising). That ends up being $25-$45/hour on average depending on the project.