Hey Raj So I know this isn't technically the question you asked but this is the answer you need.
Publishers do NOT like when authors send in manuscripts with art already done by an artist the author has picked. Publishers like to pick the artists themselves because they know what will sell, what will fit the story best, etc. If this author want to be published traditionally they should send their manuscript right now instead of hiring an artist. Furthermore I'd say if this author wants to be published but they don't even know that publishers pick the artists, then 1. They haven't done their research very well, 2. They have absolutely zero idea how this industry works, 3. Therefore there's very little chance this books ever gets traditionally published... ever.
Your best bet here is either tell this to the writer and that'll help them make better decisions in this process. Or if you're feeling less helpful, negotiate a fair work for hire contract where you'll get good money now for your work. Cause chances that this book actually gets published are slim. Chances that this book sells well enough for you to make great royalty money even if it DOES get published are even slimer.