My friend downloaded terraria with GOG and when he clicks play the game doesn't boot, please help :)

I assume when you say "GOG" you really mean Galaxy (GOG is the website, Galaxy is the client). If so, try launching the game without Galaxy. The client is completely optional and in its current beta state, it seems to get in the way more than help. If the game won't launch via its start menu or desktop shortcut, then at least Galaxy and its quirks will be eliminated as a source of the problem.

Don't forget to install dependencies as well, from what I recall, Terraria requires the XNA framework.

He downloaded the required files but it still wont open. he laptop does meet the specs for the game.

My friend downloaded terraria with GOG and when he clicks play the game doesn't boot, please help :) He downloaded the required files but it still wont open. he laptop does meet the specs for the game.

Have you contacted support? You can also send a message to JudasIscariot or other gog employee.

Did he try to start the executable from the Windows explorer? :)