Call of Duty: Modern Warfare 2
Despite the sexual harrassment she'll recieve if she pops on a headset and joins you on Xbox Live, Call of Duty: Modern Warfare 2 is a particularly bad game to persuade someone who doesn't understand games that they're not violent murder simulators.
Of course there's the issue of shooting down entire armies full of middle easterners, but the No Russian level in particular is likely to offend your mother's sensitive sensibilities. It's hard to imagine the kind of mom that's completely okay with you running through an airport, mowing down innocent bystanders. If you do happen to have a mom that's into that sort of thing, you probably don't need this guide.