If I find that my partner has difficulty to avoid the net guy constantly on returns, two things I usually try based on the situation of the match. But will never do anything which can kill his confidence, or will say anything which can hurt his ego. "ALL" of us tennis players have some level of ego
1. Ask him to hit straight at the net guy or down the line passing shot for that point (the way of saying it should not be negative hinting that it is because he is missing too many cross court, but because you want to surprise the opponent) , and I will move back few steps. Usually the partner wont take it as a bad thing, since he understands that when hitting straight at netguy I need to move back to cover a weak/lucky volley. Also this loosens him up, since now he has a target (instead of an avoidance target). And most of the time I see him hitting better shots, and outright winners, because of the gained confidence on the shot.
2. Put another surprise the opponent, change of shot of short slice angle and charge or chip and charge, instead of deep ground stroke, if that is what he was trying. Less pressure, since it is a surprise strategy, and you are willing to take risk early in the point.
In general, instead of keep trying the same thing, make it interesting by helping him mix it up with something else, and you cover it. And once that set is over, think again whether you folks are playing on the correct side on returns. If your partner has constant trouble hitting cross court from "ad" side, he probably can hit better cross court from "deuce" side and vice-versa. Identify it and change side if needed.
My partner, for whatever reason, keeps sending the ball straight to the waiting racket of the net player. Or can't hit a low passing shot when opponents take the net. Or keeps popping the ball up. If I say, "How about if I move back so I'll have a better chance of getting some of those balls back?", they will argue with me.
