Let's put it this way, she's just out of "school" and needs something to show off her work. Your game is that something.
If she's 50/50 in the business, then she shouldn't be taking anything more from you. Yes, it's nice to be able to pay someone for their work, but let's be real. If, by the end of the game you paid her $1000 for her work (purely an example), and the game makes $1000. She's made $1500 and you made -$500. How's that fair? ;) If she takes money for work done, then her share should drop. You need to be paid for your work too!
Sounds like she's only doing the graphics, whereas you're doing the coding, the website, the marketing etc.
Also remember, she'll be able to use your game in her portfolio to get more work so it's more beneficial to her.