Why remove? Just copy his code, rebadge and run both apps. You could have 2 different price points and see how it performs; that'd be interesting data.
I think you're going to have to reveal your hand first unless it's not making him much money in which case he might be willing to state a price. If he was actively shopping it around you'd be in a better position but every app developer thinks they're gonna be bought out for big $$$. Hard to get beyond that mentality.