We all have had partners like the one you describe.
In basketball we always stress the pre-game. That's the ideal time to address things, but not much can prepare you for what you describe. I ALWAYS tell the U to call anything that I miss. We're a team and need to work as such.
Now, having said that, I didn't do that once. I walked into the gym and saw who who my partner was. I hadn't worked with him, but his reputation for incompetence in every sport proceeded him. He is hands down the worst official I ever worked with... and he is terrible in every sport. He has actually been told by many schools, even Jr. High Schools, they could no longer use him any more... and that was in baseball and basketball, too. The night we worked together he looked the wrong way on every serve, watched the ball when he should have been watching the net, called a foot fault on a serve from the floor.... which he was notorious for even when they were not even close. He always argued that he was doing fine when partners tried to help him out on these things. So.... what do you do? You get in, call the games, shake hands, and say good-bye afterwards, and get out. Some nights that's all you can do. And PRAY HARD that you don't work together again, but if you have to, make the best of a bad night and look forward to your next match. When I coached soccer, I could always tell when one official was good andone was incompetent. Most coaches will see that, too.
Finally, if you are 2000% sure of an obvious ball handling I'd call it... but don't expect a bad partner to like it very much.