Currently stuck in this hell with a client, who needs a particular integration, but the company who wrote the program they need integration with refuses to give

Currently stuck in this hell with a client, who needs a particular integration, but the company who wrote the program they need integration with refuses to give up any documentation. Soooo we're just winging it at the moment, because it's their job to provide it to us (as agreed upon). I'm pretty sure they aren't going to get that integration.