Does Japan's laws have anything to say about signatures obtained under the influence of alcohol?
I know in America that "contract" would definitely not be absolute proof against a suit under those circumstances (especially with the whole premise being they knew she was dead drunk), but I never know for sure with Japan.
That said, that doesn't help if she doesn't know that, and given how skeezy Matou appears to be, it's not out of the cards that he's just counting on it being too much trouble. Plenty of dumb cruel "entertainment" programmes of such a flavour have relied on getting in only-so-much-trouble for their wrongdoings IRL. So it checks out in that sense.