Mutual Back-Slap Society
One of the great things about having a writing partner is that you can pat each other on the back, so neither of you has to risk a sprained elbow doing it for themselves.
Not everything you write is going to be pure gold, and the same goes for any human writing partner you might have. It really is important to show support and encouragement, but it’s equally important to call bullshit when necessary. Even when the offending idea is something your partner is deeply in love with. Especially then.
There are two ways to go wrong with this kind of feedback.
You might be too soft about it. You don’t want to be mean, and you don’t want to come off like you think your own work is flawless. Who are you to criticize? You’re the co-author, that’s who. If you don’t speak up, then either the defect will remain in the writing, or else later when you do finally raise the issue it’ll be that much harder to deal with. (Maybe your partner will realize on her own by then? Don’t count on it.)
The other pitfall is fixating on the things you don’t like to the exclusion of what’s good. Even if you’re not rude about it, the cumulative effect will wear your partner down. You might train him not to overuse passive voice, but you’ll be smothering his creativity while doing it. “Dull, but free of errors,” isn’t what anybody wants for a log line.
Positive reinforcement is more effective than negative. To help each other improve, and to improve as a team, you need to praise the good stuff. Offer that pat on the back, and hope to earn one for yourself.