This issue is tricky. He wants to know that he's "doing his job" in bed, but he doesn't want to know if he's not. Male ego hinged on sexual prowess, virility and skills are surely part of the reason we have Viagra for sale via mail order but male birth control is (mostly) in the testing phase.
How she can help: Talk to him... sensitively. If he's not doing it for her, she shouldn't blurt out what he's doing wrong. She should talk to him about what she like (bonus points if she says it in a breathy voice, horizontally sans clothing). The more fun/no biggie she makes things, the less likely he will be to get a bruised ego. And the payoff with what she does like will increase, because he will know and he will want to.