There are literally no costs for complying if you sell or give away a messenger app, unless you're leaving everything unencrypted or collect tons of unrelated private data, too.

If they sell private information gathered from that messenger app to undisclosed third parties, then there may be additional costs of compliance.

Maybe this developer is complaining because he's running a nefarious business model? In that case it might indeed be easiest for him to shut down his business in the EU.