I very much doubt he has an European patent on "in-app purchases", so my bet is that they threaten to sue him in the US. I'm not sure if the outcome is enforceable in Europe though.
That doesn't really matter if they can just force Apple (a US company) to pull it from the app store.
Actually, the App Store in Europe is provided by iTunes S.à r.l, a Luxembourg (EU) company, not Apple Inc., so a US court can't really force anything to be pulled from the App Store in Europe.