A developer was using open source code and some of their own GUI customization to publish an app in the App Store for iOS. It was a decent app and I really liked it. However, inexplicably the dev has removed it from the App Store and it is gone now. Is it fair, legal, and right for me to now use the same open source and my own GUI customizations to upload my own? Neither they did nor will I charge, it will be a free app. Since my app will be very similar to the previous, would it be OK, in other devs opinions, for me to do it?