We're dealing with a dozen people here, not some giant megacorp with thousands of seats to fill. When you're hiring for an environment this small, you can afford to be choosy. When hiring those first 10 guys, he should have been careful to not pick misogynist assholes - not because of liability but because those people are generally assholes to everyone around. If you have people acting inappropriately just because there's not women around, it means you've already hired the wrong people. People are probably already comfortable, it's just hard to make "I had to work with a douchebag" lawsuits.
Similarly, when hiring the first woman into an all-male group, you can afford to make sure you don't have some lawsuit-happy, thin-skinned, man-hating ultra-feminist. You're not hiring person N+1, you're hiring a team-member & considering how they fit in with your existing dynamics is just as important as the skills they bring on board.